What is an OpenID ?

An OpenID allows you to log in to multiple different websites with the same id.

Where do I get one ?

There is a good chance that you already have one. Several big web sites act as openid providers, so you already have one if you have an account on AOL, Blogger, Flickr, MySpace,LiveDoor, LiveJournal, Orange, SmugMug, Technorati, Vox, Yahoo or WordPress. Another possibility is to get an openid at an openid provider. See here for further info.