Why does my car have this smell?

When I turn on the a/c, sometimes it has a weird smell. Kind of like...mildew, I don't know. It sort of goes away after a few minutes, but I want to know why it does that. I used to have it parked in my parents garage where they smoked, so I don't know if that has anything to do with it. It's been parked in a smoke free garage for the past few weeks though.