OpenID is a unified user identification method released as an open standard that essentially acts as a single user identification system that can be used across multiple websites. OpenID is a way to eliminate multiple user accounts across different websites, which often leads to confusion on the part of the user, especially when trying to remember all the different usernames and password combinations that have come and gone. OpenID allows users to log on to virtually any website that supports the standard with a single ID, eliminating the agony of the sign-up process and simplifying signing in to any affiliate website. As of 2012, OpenID is supported by at least 27,000 sites, including Google, Yahoo, PayPal and VeriSign.
Techopedia explains OpenID
The original authentication protocol for OpenID was developed by Brad Fitzpatrick in May of 2005 while he was working at Six Apart. OpenID was soon implemented on Fitzpatrick’s creations, LiveJournal and DeadJournal. OpenID allows users to sign in to thousands of websites without having to create new usernames and passwords. This provides convenience for the user and increases security, because user infromation and passwords are stored centrally. That means if one account is hacked, the hacker won't be able to retrieve a password that can be used to hack the same user's account on other sites.