AAAAAAaaaaahhh!!! You haven't played a tabletop RPG!!! Waaaaaaaaa!!!!!
* my head a splodes! *
Don't panic, most people haven't

. Actually, they're very cool to play, as long as you find a good GM.
If you're familiar with an RPG videogame, you shouldn't have much trouble with tabletop RPGs.
The basics:
There's a rule book that contains all the necessary rules to deal with special situations like fights or challenges like climbing a mountain. There's also a dude or dudette called the Game Master (GM), although some people like the term Dungeon Master better. I just use Master. This GM makes up a story and acts as its narrator for the other players to enjoy. A good GM knows how to write a cool story, knows how to narrate it and knows the rule book well, so the gameplay doesn't slow down much with rule checking and all that stuff.
There are also players (duh!). A player first creates a Playable Character (PC), its alter-ego in the game. Unless the GM says otherwise, you (the player) usually have total freedom to make your PC the way you like. I'm a skinny and almost 2 meter tall guy, but for some strange and unknown reason I usually play as a hot chick with lots of charisma who uses her charms to get around.
In order to determine how your PC deals with the world, and to prevent you from making a superhero with the ability to fly, see though walls and talk to jellyfish, you usually throw a bunch of dice that give you a bunch of numbers. Once you have the bunch of numbers, you write them down in a piece of paper so you get a bunch of statistics, which follow some weird logical order according to the rule book. These statistics are your PCs traits: things like strength, dexterity, charisma, intelligence, etc... Depending on what kind of PC you wanna make, you'll distribute the bunch of numbers accordingly. If you have a very high number and you want to make a warrior, then the most logical place to put the high number would be either dexterity or strength.
According to the rule book, you may also get a bunch of useful skills like juggling, singing, folkloric dancing, break dancing, tap dancing, pole dancing, belly dancing, and so on. Once you're done, you should have one or more pieces of paper with a bunch of numbers and words. And that's your PC!
In order to play, a novice player doesn't need to know the rules. As a matter of fact, most of the game is just talking between the players and the GM. The GM starts narrating a story, and the players must roleplay their PCs in the story. Here's a real sample from a Lord of the Rings parody my friends and I play:
GM: You find yourselves at the edge of the Very Dark Forest, a frighttful place from which only a select few have been able to come out with very deep mind scars! You must cross it in order to get to the other side!
PC1: So, what makes this forest so terrible?
GM: Two words: forest elves.
PC2: Say what?
GM: Forest elves.
PC1: Crap, we're screwed! We better hurry and stay alert!
PC2: But why?
PC1: Because last month a groupd of travelling entertainers were assaulted by them, and only one girl survived!
PC2: And how did the girl survive?
PC1: Well, she was the only girl. You see, forest elves are CENSORED.
PC2: So?
PC1: I mean super wacko CENSORED, and totally psycho. You don't want to mess with them. They're like Michael Jackson, but super vicious. They also have a preference towards dwarfs.
PC2: What!?
GM: While you two argue, you begin to hear a tune.
PC2: What kind of tune?
GM: It goes like "In the navy.... you can sail the seven seas..."
PC1: AAAAAAAHHH!!!! RUN FOR YOUR LIVES!!!
...And no, I'm not making this up. Our GM even made a CD with all the tunes he had prepared so we could put ourselves in the mood. Of course, after 4 hours of playing we hadn't finished yet, because we were laughing so much. Forest elves are scary, really.
You usually talk and discuss the next action with the players and the GM. When something that depends of your PC's ability happens, you usally decide the results with dice. There are very different rules for every game, so if you're new to the game and don't know the rules yet, the GM will tell you what to do and what results you should get to be successful. Most of the time, you should get a number above certain value; if you get a number below the value, you fail. This value is calculated according to your PC's stats, the game rules and the situation set up by the GM.
This is the basic tabletop RPG. Depending of what you play, the game may use stuff like miniatures to recreate fights, detailed maps, cards, many different type of dice (usually called dX, X being the number of faces the dice has; a normal dice is called a d6), and pretty much anything else the GM may come up with that could be useful.
A secondary thing to consider is where you're going to play the game, and with whom. If you've never played, I recommend finding an experienced GM to have a fun time, as well as experienced players so you get the feeling of the game. As for the place, some people like to play at their homes all set up with candles, low lights, or whatever to make the room feel like the game, but I usually don't care much about that, and I've played in bars, pubs, at school, and even airplanes.
There's also LARPing (Live Action Role Playing), which is like an RPG but instead of sitting down and throwing dice, you dress up and go outside and recreate all your PCs actions. I've never LARPed myself, but knowing my friends, if we ever tried to LARP we'd get arrested

.
And that's about it. If you ever have the chance, try it! You may or may not like it, but you'll remeber the experience.
Edited: spelling and minor corretions.