I think it's a title you give yourself. Kinda like, "I'm the master of this web of pages. I'm the Webmaster!"
Or, like, I'm a web graphics specialist. Why, that could be just about anything! hehe.
As far as the people I've worked with, the webmaster is usually the top-tomato in terms of getting the servers up and running and keeping them that way, regulating content to some extent, and regular maintenance of all things web. There could be a test or some sort of official process. Guys, am I wrong?
Yah, I think webmaster pretty much means you are the one who coordinates everything on the site. But I don't think this term should be used when it's just one person and their own site. A "master" in the sense of the word means that it is someone at the top of something, and when you put that in the context of a web site, it seems to make more sense when a group of people are in a colaborative effort which involves the maintinence, content, graphics, backend, etc. Sort of like another word for supervisor, or boss. I think it pretty much means that the webmaster is the person who directs everyone else. And you're asking what it takes to be this person: A lot, I think you've got to know your stuff. Take Mark, Flashkit's webmaster. He started everything with Flaskhit, and he has people working with him. But for the most part, it's his deal, he makes the decisions, directs things, etc.
I think it would be stupid to use the word webmaster as a title for yourself when it is just you and your website.