User account
In case you're wondering what OpenID is, it's decentralized login.
LiveJournal users enter LJUSER.livejournal.com
(other OpenID suppliers)
Instead of giving me a password (maybe the same password you use on all sorts of sites!) you put in the domain name of your website. If it's OpenID-enabled, then you login there, and that site verifies your identity for me, without me storing a password, and without you having to register here.