Her: "What game is that?"
Me: "Minecraft. It's kind of a building game?"
Her: What, like Lego?!"
Me: "A bit. But you need to gather resources and food to survive as well."
Her: "So can you starve?"
Me: "Yeah"
Her: "So it's like the Sims?"
Me: "Not.. really? Here, let me show you..." -loads up the guild's Minecraft map-
Me: "This is a map of everything my friends and I have made on it. That's a farm, that's my house, this is a forest, that's the sea, this is all ice thanks to a snow related accident we had..."
Her: "So you built all this with other people?"
Me: "Yeah, there's about 10 of us. There's more but this map hasn't updated in a while."
Her: "Why not?"
Me: "The server owner's kind of busy and hasn't had time" (I didn't bother going into plugins and mods)
Her: "Oh... It looks interesting, I bet it could be a big hit if more people played it"
Me: "Well it is, there's something like 25 million players worldwide"
Her: "Wow! And he has to update all of them?"
Me: "Well no... yeah."

It's difficult to explain something to people who have to simplify everything down to comparisons. I guess Lego Sims is a very broad way to describe Minecraft... but my word, it's a pain to break something so complex down into that.