Five Easy Pieces of Online Identity, by Evan Williams:

...there are five different things people mean in different contexts when talking about identity and the Internet. (There are probably more, but these are key.) Each of these are offered as features of different services. Sometimes they are combined, sometimes they're not. And sometimes companies outsource these features to other services. With these pieces in mind, you can look at different companies, services, and protocols and realize which pieces of the identity puzzle they offer (or perhaps should).