Some file sharing sites require registration, and once you do so, a 'token' is added on to the end of any torrent you access on that site. The token authenticates you and allows you to download - if...
The most common form of 'Hacking' users accounts on websites, or computers is simply to acquire that users password. It is reasonably simple and unglamorous, very unlike anything seen in the movies....
A user account is an account supplied by a user before he/she uses a particular system that requires it for identification and authentication purposes.