OpenID is a solution for the log-in problem of having multiple identities online. With OpenID, you create one master identity online at a site you use a lot and tend to remain logged into--for instance, a social network or your personal blog. When you need to identify yourself to another new site, you point that site toward your main identity-providing site where you're already logged in. Your main site sends the new site your log-in credentials, so the new site now knows who you are.
In theory, if OpenID was adopted on every Web site around the Web, you'd need only one universal log-in and could forgo the often tedious practice of remembering user names and passwords. In recent years OpenID has seen growing competition from companies like Google and Facebook, which are pushing competing services.