Im using Janrain at the moment and dont know much about OAuth.
Could someone explain the differences between these two?
Janrain is using OpenID, so the user must get an identity with an OpenID provider. You already know how it work: User interacts with a provider that prompt for credentials. OpenID is a good solution if you want a Sign-In system that accept existing accounts from OpenID providers.
In the case of OAuth, it can be completely transparent to the end-user. OAuth works with Tokens and each token grants access to a specific site or resources, it's all about "authorization". You can also configure a token to expire after a defined duration. OAuth can also be used for Login, that's how Facebook works.
Example with OAuth: Your website use Janrain for authentication, and now, you want to allow users to import photos from Facebook, but the user provider is Google (for example). You will redirect the user to Facebook for authentication and the user will be asked if he want to grant access to your website. If the user agree, he will be sent back to your website and in background, you'll get a Token from Facebook which must be used in future communication to validate actions. At no point the user shared username and password with your site.
I recommend you to read on OAuth. You can see a really good guide here, for both OAuth 1.0 and 2.0.