OAuth 2.0: Why we need it, What are the benefits and use cases

Both OAuth 1.0a & 2.0 support two-legged authentication, where a server is assured of a user's identity, and three-legged authentication, where a server is assured by a content provider of the user's identity. Three-legged authentication is where authorization requests and access tokens come into play, and it's important to note that OAuth 1 has those, too.

more: http://stackoverflow.com/questions/7561631/oauth-2-0-benefits-and-use-cases-why