Yes, I know what it is, but never know how to answer that question. So I always answer "It's like a cheap hooker, it'll be what you want it to be".
Well it's a lot easier to say than...
Minecraft is a building game that has monsters in it and you have to mine the materials to build things. Or you can have everything and no monsters, so you just build. It's a single person multiplayer game....... well you get the jist.
So how do you answer the question, What is Minecraft?