Sim means "simulation". In Second Life, a sim is an area of virtual space (land, just at the ground level - under the ground and in the air don't count towards square meters of land, as far as I know). I think it's the amount of virtual space that can be supported by one server, although I could be wrong about that.

Scrump, I'm not qualified to answer that question. I met my husband online, and that's when I actually got something resembling a life.

One thing Second Life can do for musicians is make it possible to have performances that are heard (and in a sense, seen) by people all over the world at the same time. And some performers get paid for their gigs in Second Life.