The OAuth protocol
Before the introduction and extensive update of OAuth, each service with which you needed to interact provided their own authorization protocol. These methods diverged significantly. Every time you wanted to make use of a new API, a new authorization scheme had to be learned and implemented. This made interacting with a significant number of services very difficult.
OAuth was created to solve the problem of a lack of standardization around authorization with different sites. The creation of Version 1.0 took about two years and was contributed to by a number of major industry players, as well as smaller interested parties.
There are currently two major versions of OAuth in the wild: 1.0a and 2.0. Version 1.0a is a ...