OAuth is an open standard for online authentication. It enables a user who stores information such as a password on a particular Web site to then authorize yet another site to access that data, whereas the user's identity is never shared with that site.
It allows developers to create third party apps that can access a user’s account information without requiring them to hand over their log-in credentials. Before now, Twitter applications have required users to enter their usernames and passwords.
Twitter's OAuth interface is now open to all developers, enabling more secure access to the service via its application programming interface from third-party Web sites according to an announcement by Twitter's API leader, Alex Payne.
Twitter's OAuth Now Open To All Developers