fuck acronyms - i still don't understand what an rpg is - i had it explained to me and it just seems to be 'any game that isn't a fighting tournament type game or a racing or sports game'
Since others have asked previously, the C stands for Computer, to distinguish it from the pen & paper (or should I say character sheet), and multi-sided dice (often a whole box of different shaped dice) format that originally defined the games (often under brand names such as Dungeons and Dragons, Middle Earth Role Playing etc. for fantasy versions, there were also sci-fi and alternate world formats) that are, have, and will be played by groups of people the world over.
RPG - Role Playing Game (obviously!) - the defining factor is really about character development, you decide to play a particular character with certain skills, characteristics, and morals - and then play the game true to how you think the character you have made up would behave. The player character is central, and the aim is to play the role you have chosen - a bit like acting.
There can be (often numerous) shoot-em-up elements in an RPG game (although true to Dungeons and Dragons role playing it's often turn based - one turn lasts a certain amount of time and allows you to perform one action or part of an action - the best computer RPGs let you choose between turn based or FPS style combat), but it shouldn't detract from the main thrust of the game, which is for your character to become the type of character you want them to be. In a good RPG this will impact heavily on the storyline and resolution of quests (which should be able to be resolved in numerous different ways depending on your character's morals, or ignored, as you see fit).
Where an FPS will for example put you in the role of a soldier whos aim is to take command of an enemy military base, an RPG will let you decide which side you want to take, and with some consequence if you act out of character - and the game will develop accordingly.
It's a format that doesn't always translate well into computer games - in the original pen & paper sitting around a table with a can of beer format you had a person who ran the group who could make decisions on the fly about how character A's actions impacted on the plot both in the longterm and short-term, and could devise stuff on the spot - obviously a computer game is always going to be more limited, because not every action can be programmed for in an interesting and novel way - which is why freeform games where you can just wander all over without being limited to a certain number of locations, such as Fallout 3 and Morrowind are so popular.
When a game manages to do it well, with freeform exploration and multiple different possible interactions and multiple endings depending on your actions (if you even bother to follow the storyline), it is quite a triumph of programming and a very big project, the aim being to make a world where you can do anything and go anywhere, with any set of morals, and any or no plan - which is why those games score high on my scale, because they plonk you as your chosen character down in a fantasy world without dictating to you who you are, what you have to do, how to do it, or where to go next - in all of those games you can just say bollocks to the plot (thanks for writing it, I may come back to it in a month or so), I'm going to wander out into this landscape, find a lake*, go fishing and trade fish for medicine and weapons. Until I get bored with that and want to do something else.
*or alternatively, decide to break into people's homes at night, murder the occupants, rob everything they have, and become leader of some criminal faction.
*or alternatively, find work at the nearest tavern, make an honest living, and buy a humble abode somewhere.
*or alternatively, go and find your father/slay the dragon/cure the plague/put down the rebellion/support the rebellion/save the world/destroy the world.
It's the "or alternatively"s along with the central role of the character that you decide to play that makes an RPG an RPG.
I'm looking forward to heading over there. 
