oAuth

oAuth (Open Authorization) is an open standard for authorization. It allows users to share their private resources stored on one site with another site without having to hand out their credentials, typically username and password.

OAuth allows users to hand out tokens instead of credentials to their data hosted by a given service provider. Each token grants access to a specific site for specific resources and for a defined duration. This allows a user to grant a third party site access to their information stored with another service provider, without sharing their access permissions or the full extent of their data.

OAuth is a service that is complementary to, but distinct from, OpenID.

OAuth is the preferred authentication method for APIs. There are plenty of resources available for deploying an OAuth server in your preferred language, for use with your API.