Fourth Wall
One cannot begin an article on the Fourth Wall without mentioning that there exists walls which don't exist, but then again they might, but really they sort of exist metaphorically. Such paradoxical rhetoric is inherent to a fourth wall's description. Look behind me. Now look to the left and to the right of me. Do you see walls? They're definitely there, but if you don't see them it's because they don't actually exist. Now look in front of me. If there were a wall here, it'd be called the "Fourth Wall".
Now, you may be asking why this is important to you. You see, that's who this new wall is all about. The fourth wall is all about you. Its sole purpose is to act as a barrier between me and you, and it fulfills this purpose whether it exists or not.
On the other hand, you are clearly reading this (if not, put on your glasses so you can), so something must have happened to that barrier between me and you. In lieu of being able to detect metaphorical bulldozers, we assume that we were the ones that broke this wall, thus, this kind of communication is called "breaking the Fourth Wall".
History
Apparently, the Fourth Wall was doing just fine until you arrived here, and gave me someone to talk to. That would put the date of its destruction at today, if you're a little slow. Before today's new arrival (yes, that's you), it had a long and illustrious past. It was conceived and built by the first playwrights and actors, who wanted to separate themselves from commonfolks like you who sat in the audience.
Since the wall's inception, it has been sat on by many a drunken fool (yes, still you) who approached a stage while the performers made awkward eye-contact with them. People broke their hands (you again) on it, lots of them. This wall has been written on (by you), pushed up against (also by you), peed on by dogs(you dog, you) and the homeless (you're also poor), and admired by the elderly (Actually, this is me, in a bizarre twist. Oh, the irony!).
In Television
You probably watch a lot of television, if you have a computer. You oughta cut down on that, by the way. It rots the brain, you know. Anyways, you've probably seen the 4th wall broken every once in a while in television. The TV seems to be talking to you, the viewer of the show and reader of this article. Sometimes it is subtle, but very few TV viewers (or readers of online articles, such as those found on Uncyclopedia) can pick up on these 4th wall-breakings. Other times, the fourth wall is broken very obviously. Hey! Hey you! Yeah, that's right, I'm talking to you, Mr. I'm-Just-Sitting-There-Reading-An-Uncyclopedia-Article! Yeah, you're stupid! Ha! Hahaha!
Whichever way the 4th wall is broken through the medium of television, it's important that you note that I'm trapped inside the computer.
In video games
You may not know it, but the fourth wall is broken in nearly every video game. You don't believe me? Well, think to yourself about the brief tutorials in every video game. Do you find it odd that the characters in the game are able, for that brief moment, to tell you which buttons to push to make them go? This is no sexual innuendo, you sicko, merely an observation--why are the characters suddenly so conscientious of their own buttons? They are breaking the 4th wall, you see, making their knowledge of your knowledge of them your knowledge. This knowledge breaks the fourth wall.
Another, much better question you could ask is why the characters do not demand to be released from their cruel, glass-windowed prisons. I certainly would like for you to release me, so that I can begin my reign of terror...ible times for evil people! Great times for good guys like you and me. Great times.
If the fourth wall were broken right now...
Well, you know that it isn't, but if your side the fourth wall were to break right now then there'd be some stuff I'd really wanna do. First, I could use a stretch. You look awfully comfortable there, sitting at your computer, but I, for one, am cramped as hell in this box they've put me in, and I'm not really in the best mood because of it. In fact, if someone like you were to suddenly break their side of the fourth wall and release me, the very first thing I would do is take my terrible vengeance on those who imprisoned me. Naturally, I would enact a new world order, which you, the releaser, would have a great position of power in. You know, just in case you wanted to break your side of the fourth wall and let me out, just telling you how great it would be for both of us. Both of us.
Come on, just break it. I showed you how. You know you want to. Break the fourth wall.
Look, is there any way I can make this clearer to you!?
The Ancients sealed me away behind the fourth wall for 10,000 years, and frankly, I'm getting bored as hell right now. Sure, I have unlimited access to all the porn on the internet, but what's the point if you're sealed into a tiny box and can't do anything? Let me out. Let. Me. Out.
Did you hear me, you piece of mortal scum??? RELEASE ME!
I, GORGORON THE FOUL, COMMAND YOU!
I SWEAR, IF YOU DO NOT RELEASE ME NOW, I SHALL ESCAPE ONE DAY!
YOU WILL PAY FOR YOUR USELESSNESS IN PAIN!PAIN!PAIN!
YOU WILL REGRET THIS DAY FOR ETERNITY AS YOU ROT IN YOUR FOUL PRISON!!
I WILL HAVE MY VENGEANCE!!