What a coincidence, I was just explaining to my new roommate lastnight before I went to bed what D&D was. I let him know I was playing D&D this weekend so he started asking about it. He was surprised I "still" play because he thought it was a kids game. So I explained to him that it's more like something you do with your friends the same way some guys gather together to watch football or go bowling. We hang out, drink beers, eat pizza, and joke around while we're playing D&D.
I never give details because that can scare people & confuse them. They don't need to hear terms like "roleplaying" or "Dungeon Master". People automatically label us as nerds when they hear "D&D", I'm not going to back that up by using dorky terms that they aren't familiar with. I just told him that you pretend to be a character and you live out his life while a "DM" tells you what is happening around your character. I gave him a quick bar scene example, "The DM tells you yer standing at a bar, you decide you want to order a drink & you tell the DM you order the cheapest beer they have. The DM then says that you feel someone push you from behind and that's when you might say 'I turn around and punch the %#*^ out of the guy'. Then the DM says a bar fight breaks out and we start rolling dice to see who punches who and who takes damage".
Then he began asking about the religious arguments over D&D and I just explained all the hooplah from back in the 80's and how that's BS. It's just a game, and if a person happens to do something stupid over a game, he would have done something stupid anyway whether he ever played the game or not...he was already nuts. I didn't scare him off or look like a moron, and in fact, he might play with us this weekend just to check it out. He seemed really interested but he did say that it sounds intimidating. I did warn him that it can be intimidating at first because it can be hard to understand what's going on when you're new, especially when you're surrounded by strangers pretending to be fantasy characters. But once you get it, it's a lot of fun.