Black holes appear frequently in fiction, and rightfully so: they're the red-headed step-child of the universe. But more often than not, they are attributed near-magical powers that have nothing to do with black holes. And before I can talk about what black holes aren't, I need to explain what they are. First I'll need to talk about the gravitational force and "escape velocity." I'm going to get a bit basic for a little bit, so if this is review for you, bear with me. For any two objects with masses m1 and m2, and gravitational constant "G," the gravitational attraction force between them is: $$ F(m_1,m_2) = G {m_1m_2 \over r^2}.$$ To review, this means that as you get farther away from the center of mass of an object, the pull it has on you drops off rather quickly.
So let's say you started out on the surface of a planet and want to figure out how fast you would have to be going straight up in order to completely escape from its gravity well. You can figure this out by solving for the "escape velocity." You do this by integrating the above equation to figure out the gravitational potential energy (not shown), setting your gravitational potential energy equal to your kinetic energy and solving for v. This gets you to the following equation (I've skipped a few steps for simplicity): $$ v_e = \sqrt{2GM \over R} $$ Where "M" is the mass of the planet you're trying to escape and "R" is the distance from its center of mass that you start at (e.g. if your example is the Earth, then R would be the radius of the Earth). So what this means is that the more massive a planet is or the closer you are to its center of mass, the faster you would have to be going to escape from it. This makes logical sense.
Now let's take this to the extreme. There is a speed limit in the universe: the speed of light. So for an object with mass M, how close to this object's center of mass would you have to be in order for your escape velocity to be the speed of light? Just rearrange the above equation and you get: $$ R = {\ 2GM \over c^2} $$ Where "c" is the speed of light. This radius has a special name. It's called the Schwarzschild radius (pronounced "schwartz-shield"). Every object has a Schwarzschild radius, but here's the kicker: for a spherical object, the object's entire mass as to be within the Schwarzschild radius for you to experience a scenario where the escape velocity is the speed of light. Meaning I can't just dig down into the Earth until I'm 8.7 mm from its core (the Schwarzschild radius for the Earth) and expect something special to happen (other than getting completely crushed by the weight of the Earth on top of me). What this means is I would need to compress the entire Earth into and incredibly dense ball 8.7 mm in radius, and then I would have achieved my goal: a tiny little black hole. If you're curious, click here to play around with a Schwarzschild radius calculator.
That's what a black hole is: an object so dense, its escape velocity equals or exceeds the speed of light. Here's where things get a bit crazy. What we think of as the "surface" of a black hole, isn't really its surface in the typical sense. Picture a black hole in your mind. The "surface" of that black spherical object (right at its Schwarzschild radius) is the poetically named boundary called the "event horizon." Light or a signal of some kind outside of the event horizon can escape the gravitational pull of the black hole, while just on the other side, any kind of signal or light gets trapped since the escape velocity exceeds the speed limit. But... then what happens to the light inside of a black hole? Where exactly <i>is</i> the object inside that created it? Does it fill the whole space, or just the center? There are many theories, but they're all just theories without a robust theoretical or experimental basis. Really, no one knows. Inside of the event horizon, our understanding of physics breaks down. Time and space are so distorted that they cease to be recognizable by any set of equations we have. Past that boundary, "events" as we understand them no longer exist (which is why it's called the event horizon).
So the black hole that we see is really the universe's cloak around something unknowable. It is not a planet, or a star, or something that you can land in or on. The event horizon is the border between the universe we understand and a realm so foreign that it defies logic or reason. However, nothing that exists in nature exists outside of the rules of nature. So therefore there have to be rules that govern it and answer the question of what's inside, right?... Well, probably. The problem is that our (very thorough) understanding of general relativity and quantum mechanics don't mesh well. Both areas of study have been around for a hundred years and have been proven over and over again. General relativity deals with very large scale objects and how gravity distorts the fabric of spacetime. Quantum mechanics is a statistical model that deals with very small scale objects and their interactions. Normally, these two arenas of physics don't overlap at all. But inside of a black hole, things are very different: gravity is so intense that it would affect the quantum makeup of reality. In order to fully understand this, we would need a mathematical model consistent with all of our quantum mechanical and general relativistic observations for the last century, but would also be able to successfully mesh the two in this extreme case. A so called "quantum theory of gravity" has yet to be discovered. And so for now, black holes remain a celestial black box.
That's what black holes are. Now what are they not? They're not portals. They're not time machines. They're not a way for you to magically communicate with your earlier self (ahem, Interstellar). They have nothing to do with dark matter (look for a future post on dark matter). And if you got anywhere near one you would most certainly be dead. The gravitational effects are so unimaginably intense that as you got close to the event horizon, the gravity acting on your body parts closer to it would be much much greater than on your body parts farther away. The resultant condition actually has a name: "spaghettification," which is exactly what it sounds like, and not something you want to experience. So in summary: black holes are exotic spherical regions in the universe where our understanding of the laws of nature no longer apply. And while some creative license is ok, Hollywood frequently treats black holes as a convenient deus ex machina that can take that license to the breaking point.