What is Open ID?
Open ID is a way of telling a site you want to interact with this is me, over here on this other site, so now that you know who I am, you'll let me comment or log in, or do whatever that site might allow for Open ID users. A lot of sites you might have an account with act as source sites for an Open ID. Livejournal, Dreamwidth, InsaneJournal and services like Yahoo! and Google are just some of many that have this function built right into the account you may already have with those providers.
You are likely familiar with Open ID if you read or post at Dreamwidth a lot. You will see Open ID names attached to imported comments, to new comments, and you've likely seen links to tutorials on setting up an Open ID account to use on Dreamwidth.
What you don't see as much of, especially not in fannish locations, is Open ID used on Livejournal. You will see a growing number of people commenting with their Twitter or Facebook account names, and the farther you get from transformative works fandom and into professional authors' journals or other kinds of fandom activity, the more likely you are to see that.
( read more about why you might want to use Open ID and how to set it up )