Let's get this out of the way: OAuth can be somewhat difficult to grasp. Adding to this fire is the fact that the OAuth framework/protocol has gone through a major revision in the last few years. Version 2 was published in 2012, but due to a variety of factors, there are some web applications that continue to implement the OAuth v1 protocol.
Note
OAuth 2.0 is not backwards compatible with OAuth 1.0. Moreover, OAuth 2.0 is less of a formal protocol specification and more of an authorization framework specification. Most OAuth 2.0 implementations across modern web applications are not interoperable.
For the sake of simplicity, we'll view a high-level overview of the general terms, vocabulary, and functionalities of the OAuth 2.0 authorization framework. Version 2 is the simpler of the two specifications and with good reason: one of the design goals of the latter was to make client implementations simpler and less prone to error. Much of the terminology is similar, if not identical...