The incomprehensibility of OpenID

Whenever I see OpenID in use, it confuses me. What does it mean? How does it work? It isn't intuitive. This is why I never sign up for, or log into a site which uses it.

One site that strangely insists on using it as the only method of authentication, is Stack Overflow. Often I have been tempted to participate, but the lack of an understandable form of authentication prevents me from doing so.

But today, there was a question about Stackless there. I gave in and clicked on the OpenID buttons. Stuff happened, I don't particularly understand what, and suddenly I am authenticated. I'm still confused.

Creating an account with a username and a password, or even just a password linked to your email address, is simple and comprehensible. Whatever this is, it is neither of those things.