This is the system that allows authentication using URL/domain based IDs that give the Internet user the reins in terms of using various IDs that they have control over the security, passwords, and even administration.
An OpenID is a URL that can be used as an ID or log on/password system across unrelated sites where the Internet user controls which ID to use and the security is based on the domain of the URL. This system of authentication puts a great deal of power into the hands of the user and can free the various sites from untold security and password administration. Also, as the system is based on URLs, it fits well into the Theory of the Web as One Big Database, using a universal and unique ID for each ID (not a tautology).
See more at OpenID