I've heard a few stories about it, but I haven't joined. I think of it as a place where people can carve out a place for themselves. You can make up a persona, purchase a piece of "land," design it how you like. You can try to get people to visit your place, you can visit other people's places. You can try to make money.

People with artistic and technological talent have made money by selling "costumes" or bits and pieces of costumes for your avatar.

People who have sex-type places make money on there. Sex chat, sex talk. Sex sells. lol.

As others noted, regular businesses also have a presence there. You can hold a conference, give a lecture.