XAuth is a Twitter API method that allows easy OAuth access token retrieval for an account without going through a full OAuth authentication sequence.
With the XAuth API method you can supply Twitter credentials (username/password) and get back a usable access token in one go.
This might be a better functional match for desktop or mobile (=non-web) clients. It could also be used for a one time conversion of a userbase from Twitter credentials to Twitter OAuth access tokens.
See the documentation by Twitter about XAuth for more information. You need to request permission to use this method for you Twitter application by emailing Twitter, stating why you would need this Use Case for your application.
