At the Eiglophian Press blog, G. Benedicto asks us how we would explain D&D to the uninitiated. It's a recurring topic. Most of the approaches I've seen over the years focus on comparing D&D to some kind of story/drama format, from "it's like cowboys & indians, but with more rules," through "it's like telling a story, but with dice," to "it's like improv theater, with dice". Each of these explanations has some potential pitfalls, if taken too literally.
I think there's something more general that can be used in comparisons, something that's relevant to most adults: "what would you do?" People at bars, at parties, or just otherwise hanging out with friends sometimes play little scenario-games, like "What would you do with a million bucks?" or "What would you do if you were stuck on a desert island?" or "What would you do if you were the president?" There's also some favorites that don't start with that stock phrase, like "If you were a superhero, what would your power be?" or "If you were a bank robber, how would you commit the perfect crime?" Sometimes, it's not even about you: "Who would win in a fight: Superman or Green Lantern?"
Usually, it's just one question and one response, but sometimes someone will counter your response with "But what if you can't find coconuts on the island?" That's what a GM does, in an RPG. And what the dice do is inject some surprise, or decide whether you find coconuts, or tell you if you finish building a raft before the monsoon season. And what D&D specifically is, is an extended game of "What would you do if you were a hero or wizard, like in stories about King Arthur or Hercules, and you were exploring the lair of some monster, looking for treasure?"
But with dice.