Second Life is basically an online Sims. Real people go there to do real-life things- people actually make money on it, but by no means do you have to do that. I go there to roleplay, mostly.
CJB- I have a friend who is blind and plays on Second Life. She says it's difficult, but invaluable because it gives her a good social outlet. A lot of autistic people go on there to be in touch with the world because of their disability/syndrome. Physically disabled do the same.