What is a blog?

What's a "blog", a.k.a. "weblog"? Here is a little article on Ask Yahoo about it...

"......Weblogs and online journals are often confused, and they can frequently overlap in content and style. But purists point out that a person writing in an online journal or diary is logging their life, not the Web......"

Since I have linked to that page and noted it, this is not just an online diary, but an official blog ; )