The problem with explaining role playing games is that everyone is different. More so that movies, books or other such media, a roleplaying game can be what ever you make it. I have friends who will flatly refuse to play any game that actually involves dice rolling, whereas others still like the dungeon bash style games which are essentially elaborate board games. Some like melodrama, others like politics, others simply like the problem solving. All of these aspects can be included.
To answer the original question, I would start by sitting back and thinking about what you get out of the game and explaining that (like godrik did with his excellant explination of fashion, which taught me some things). Then attempt to explain that the style of a game reflects the people playing it and the person running it. A game about child-like fairies is going to be quite different from a game about spy vs spy action at the height of the Cold War, which is going to be different from a classic D&D dungeon bash. Just as a horror film is very different from an action film.
To me it is all about the wide variety of human interaction, reguardless of genre, setting or style of game, so I can enjoy nearly any type of game.