OpenID is a set of protocols for user-centric digital identities. These protocols allows users to create an identity online, using an identity provider. This identity can be used on any site that supports OpenID. Using OpenID-enabled sites, users do not need to remember traditional authentication tokens such as usernames and passwords for each site. All OpenID-enabled sites accept a single OpenID identity. This identity is typically a URL. It may be the URL of the user's personal page, blog or other resource that may provide additional information about them. That mean a user needs just one identifier for all sites he or she uses. services. OpenID is an open, decentralized, and free user-centric solution. Users may choose which OpenID provider to use, or even create their own personal identity server. No central authority is required to approve or register OpenID-enabled sites or identity providers.
For more information about OpenID visit the OpenID official site.