Because I know clearly what one is, but I am unsure what the other one is. To me, the URL is a webpage. And it's not clear to me how a webpage is going to log m

Because I know clearly what one is, but I am unsure what the other one is. To me, the URL is a webpage. And it's not clear to me how a webpage is going to log me into another webpage if I am a user. Users don't see URLs as URLs, they see them as pages, like on a newspaper. So OpenID is based on a concept that alien to users.