Many people have the idea that black holes are maliciously traveling through the universe, indiscriminately "sucking" everything up. In reality, a black hole is mass that has become sufficiently compressed so as to prevent anything within a certain spherical volume around it from escaping. The gravitational effect of black holes on the universe outside them is to pull things toward them with exactly the same gravitational attraction that their mass had before it became a black hole. While it is true that the matter falling into a black hole will not return to the outside universe, the black hole attracts that matter with no more force than it did prior to becoming a black hole. Black holes are all spherical, with a boundary, called the event horizon. If you are outside the event horizon you can circular around the black hole without too much danger. Also, the mass of the black hole will be the same as the mass of the object that created it (typically a massive star).
I would like to give a quick disclaimer; observation of black holes are limited, so any discussion concerning their anatomy, or any discussion about anything having to do with black holes for that matter, is based on prediction made by the equations of general relativity. When we say, "black hole", what we are referring to is a singularity (defined to be a region of spacetime that we don't understand) and a spherical volume around that singularity where the effect of the mass and energy that has entered the singularity prevents anything from returning to normal space and time. The outer most limit of a black hole is called the event horizon, whose distance from the center of the black hole is the Schwarzschild radius. The size of a black hole is solely determined by the mass of the singularity. The greater this mass, the larger the Schwarzschild radius. There is nothing at the event horizon; It is not a physical boundary or surface, but the point at which not even light can escape the gravitational pull of the black hole. So, if you are traveling along in your spaceship and cross the event horizon of a black hole by mistake there is no way for you to escape the gravitational pull of the black hole and you will fall into the singularity.
There are two different kinds of black holes predicted by general relativity. A Schwarzschild black hole is a black hole that does not rotate and general relativity predicts that its mass collapses to a point of infinite density and zero volume at the center of the black hole. The rest of the volume, between the singularity and the event horizon, is empty space.
A Kerr black hole rotates. The rotation occurs because the matter that created it had angular momentum, and because angular momentum is conserved, much like a spinning ice skater tucking in his/her arms, Kerr black holes created from the collapse of the cores of very massive stars should rotate very quickly; about 1,000 times per second. The rotation causes the matter of a Kerr black hole to collapse into a ring-shaped singularity of infinite thinness orbiting around its center. Equations also indicate that Kerr black holes have a donut-shaped region outside the event horizon in which matter cannot remain at rest without falling into the black hole. Called the ergoregion, this region is a volume of spacetime that the rotating black hole drags around as it rotates, much like batter is dragged around in a blender.
Traveling through a black hole would essentially ruin your whole day. Imagine being in a spaceship orbiting at 1000 Schwarzschild radii (15,000km) from a 5 solar-masses black hole. The Schwarzschild radius is the distance from the center of a black hole to its spherical boundary outside of which is normal space. At this distance from that size black hole you can safely orbit the black hole because the only effect it has on your craft at that distance is its gravitational attraction. This attraction is exactly the same that the mass of the black hole would have if that same mass were in a normal object, like a star.
To investigate the effects of the black hole further you launch a cube-shaped probe with a clock attached to it. From the time of launch to until about 100 Schwarzschild radii (1500 km) the probe descends normally and the only noticeable effect is that time measured by the probes clock slows down slightly. Once the probe reaches 100 Schwarzschild radii, however, some strange and violent things begin to happen.
As the probe gets closer to the event horizon of the black hole, it begins to respond to the tidal effects of the black hole. The face of the probe closest to the event horizon receives a greater gravitational pull than the face farthest away. This causes the probe to stretch. When the probe gets within a few Schwarzschild radii of the event horizon the tidal forces are so extreme that the probe will elongate violently. The face of the probe closest accelerates very rapidly away from the rest of the probe. Essentially the probe will be violently torn apart by the increasing tidal forces of the black hole.
If the probe could somehow withstand the black holes tidal forces it would encounter another strange effect. As the probe falls towards the event horizon the clock attached to it will noticeably slow down as observed from your orbiting spaceship. This time dilation is so great you will see the probe come to a halt just above the event horizon and its clock will stop completely. However, if there was someone in the probe they would not feel any slowing of time and they would continue to fall towards the singularity where they will be crushed. All the while their friends on the orbiting spaceship see them frozen in time.
General relativity does predict connections between a black hole and what is called a white hole; the connection between the two being a wormhole. There is a certain kind of black hole, called a Kerr black hole, which rotates. The rotation makes the black hole's singularity, which is where all the mass of a black hole is, an infinitely thin ring orbiting between its center and its outer boundary, called the event horizon. This is in contrast to a Schwarzschild black hole whose singularity is an infinitely small point at its center and does not rotate. A singularity is where the mass of a black hole is. The argument is that the singularity of a Kerr black hole could make it possible to fall into it and not hit the singularity. The interior of the rotating black hole might meet a corresponding white hole in such a way that something that fell into the black hole would come out of the white hole. Though this is predicted by Einstein's equations of general relativity, astrophysicists don't believe the equations to be correct in regards to wormholes. If white holes existed, astronomers would expect to see them as fountains of energy emitted from nothing in space. Such phenomena have not been seen. Asserting that the equations are wrong is summarized in a concept called cosmic censorship, which says that nothing can leave a local region of space that contains a singularity.
Black holes actually evaporate. A black hole converts its mass/energy into real particles in normal space outside it by a process called virtual particle production. This occurs because it is possible for pairs of particle, called virtual particles, to spontaneously appear. In normal space and time these pairs annihilate each other in a tiny fraction of a second. Such virtual particles are being created by the billions every second in your body! We know virtual particles exist because some have been made real in a laboratory. This is done using high-speed particles in particle accelerators which slam into the virtual particles and separate them before they can annihilate one another. The mass/energy given to the particles to make them real comes from the particle that separated them, which loses an equal amount of mass/energy. A pair of virtual particles always consists of a particle and its anti-particle, for example an electron and a positron (an electron with a positive charge). The two particles are always identical except for having opposite charges. Normally, a pair of virtual particles annihilates each other and disappears very quickly. However, if a pair of virtual particles were to appear just outside the event horizon of a black hole something similar to our laboratory experiments occurs. If one particle was created a little bit closer to the black hole then the other, and the tidal forces of the black hole were strong enough, the pair can be pulled apart before they annihilate each other. This makes them real.
One of the particles will always fall into the black hole, but sometimes the other particle will have enough energy to escape the pull of the black hole and fly off into space. The creation of the virtual particles causes a void in space where they were made real. The black hole fills this void with its own energy. The energy used is converted from the black hole's mass and is emitted as gravitational radiation. Essentially, the black hole's mass is being converted into energy and radiated off into normal space.
Before we go into black holes it will be helpful to review some concepts regarding general relativity. Firstly, space and time cannot be described separately because both are essential in describing the position, motion and action of any object. Collectively, we call space and time spacetime. General relativity describes that spacetime changes shape, or curves, in the presence of matter or energy. This distortion in turn affects the matter and energy that come in contact with the curved spacetime. The greater the mass of an object, the more it curves spacetime. One of the effects that matter has on spacetime is that time actually slows down in the presence of matter, and the greater the mass of an object, the slower time passes near it. For example, imagine you are standing on Earth with a watch. You then send one of your friends to the Moon with an identical watch. Your watch will tick measurably slower than your friend's watch. This is because the Moon has less mass than does the Earth, and so the curvature on spacetime created by the Moon is less extreme. When your friend returns to earth, and you compare the current times on your respective watches, you will find that yours will show a time a fraction of a second behind your friend's watch!
Now back to black holes. A black hole is an object that has matter concentrated in an infinitely small region of spacetime, called a singularity. Surrounding the singularity is a volume of spacetime that is so distorted that it is impossible to leave and return to the rest of the universe. This volume is a black hole. In other words, the curvature of spacetime is so extreme that even light, the fastest moving thing known, cannot escape from it. We said before that the more mass an object has the slower time goes by near that object. If you get close enough to the event horizon of any black hole, the curvature of spacetime is so extreme that time will stop completely, as seen by someone far away. With that being said, general relativity is just that, relative. Your perception of this "frozen in time" affect is relative to where you are in regards to it. So if you were out in space, a safe distance from the black hole, and sent a probe with a clock attached to it, towards the black hole you would see the probe seem to hover in space and the clock stop ticking once it got close enough to the event horizon. Alternatively, if you yourself were to travel close to the black hole in your spaceship, you would perceive no change in time. You would continue to fall into the black hole and there would be no perceivable slowing of time. The gravitational attraction of the black hole would, however create such a great tidal effect on you that you would be pulled apart.
Some galaxies contain quasars, among the most energetic objects ever discovered. Quasars can emit more energy per second than all the stars in a typical galaxy combined. The power comes from a supermassive black hole at the center of a quasar-containing galaxy. As gases and other matter swirl into the supermassive black hole they form a disk around it. Spiraling inward, the gases are compressed and thereby heated to extreme temperature. Some of this gas enters the black hole. However, only a finite amount of matter can enter the black hole at any time. Some of the hot gases expand outward above and below the disk. These extremely hot gases escape at very high speeds, while emitting a variety of radiations.
A pulsar is a rotating neutron star. If the neutron star has a magnetic field that is not aligned with the star's rotation axis, the field emits radiation that rotates around like a light beacon. If Earth is aligned with one of the neutron star's rotating fields, we see flashes of light, radio waves, and other types of electromagnetic radiation. These flashes are the signature of a pulsar.
A nova begins with a white dwarf, which is the carbon/oxygen remnant core of a star with less than 8 times the mass of our Sun. White dwarfs form after such a star has fused (that is, converted) all of its core into carbon and oxygen. This material cannot fuse into anything else. After forming such a core, stars like the Sun shed their outer layers in wimpy explosions called planetary nebulas. If such a star is isolated, its carbon/oxygen core is destined to radiate its stored energy and cool off for billions of years. If this star was part of a close binary system, then after the planetary nebula, the carbon/oxygen core can pull gases off of its companion star. These gases fall onto the core, creating a shell of hydrogen on its surface. As more and more hydrogen is accreted into this hydrogen shell, the carbon/oxygen core's gravity compresses it, and so the shell continues to heat up. Once the shell reaches 10 million K, the hydrogen in the shell begins to fuse. The gravity of the white dwarf is not sufficient enough to keep the fusing gases in place so the entire layer of hydrogen blows out into space violently. This is a nova. Seen from earth, a nova can make a star in the sky suddenly appear between a thousand and a million time brighter than normal.