Most websites have a terms of service - also known as a user agreement. The terms of service represent what the user of a particular website accepts in regards to the reality of that website - restrictions, ownership, liability, etc. Terms of service are the law of the land for any particular online destination.
Terms of service are often quite long and quite boring. They are full of legalese and are rarely thoroughly read by anyone. They often require explicit agreement by the users and have very interesting guidelines. Because few actually read the terms, few actually care. But as the Internet continues to grow into the only important global communication and commerce infrastructure, we're all going to start caring a lot more about the terms of service.