OpenID is an open standard and decentralized protocol that allows users to be authenticated by certain co-operating sites (known as Relying Parties or RP) using a third party service. Users can log into multiple unrelated websites without having to register with their information over and over again; Several large organizations either issue or accept OpenIDs on their websites according to the OpenID Foundation
Users create accounts by selecting an OpenID identity provider, and then use those accounts to sign onto any website which accepts OpenID authentication. The OpenID standard provides a framework for the communication that must take place between the identity provider and the OpenID acceptor (the "relying party"). An extension to the standard (the OpenID Attribute Exchange) facilitates the transfer of user attributes, such as name and gender, from the OpenID identity provider to the relying party (each relying party may request a different set of attributes, depending on its requirements).
The OpenID protocol does not rely on a central authority to authenticate a user's identity. Moreover, neither services nor the OpenID standard may mandate a specific means by which to authenticate users, allowing for approaches ranging from the common (such as passwords) to the novel (such as smart cards or biometrics).