Standard - for general use, small, medium, large & x large avatars, you can add custom clothing to your avatar but you can't add other objects

Anything Goes (AG) - if you want to use mini or micro sized avatars or modify your avatar (eg add a car or chair to it)

BDSM - for those that like that lifestyle, although they are not constrained to that dim

Furry - good question - I understand it's for those who like to dress up as animals - but I'm sure someone will correct me if I'm wrong.....