OAuth is similar in many ways to Yahoo's Browser-based authorization (BBAuth). Like BBAuth, your Users must authorize your access to User data. Also like BBAuth, OAuth is token based, with one temporary token used to first allow authorization and another token that contains the User's credentials.
You can think of OAuth as a standards-compliant method for achieving what BBAuth does. While BBAuth is a Yahoo-specific method for authorization, OAuth is widely supported by a number of companies, so you won't have to learn another proprietary method for authorization. Another key difference between BBAuth and OAuth is that OAuth supports applications not only on the Web but also on the desktop and mobile phones, whereas BBAuth only supports Web-based applications.