A "blog" is short for "web log". People who think that other people want to read the blatherings of their disorganized minds, (and, inexplicably, they do) post those same blatherings as "blogs," and the blatherers are known as "bloggers."
"Blatherings" is a little unkind -- it's a word that describes meaningless and trivial speech or writing.
Most blogs are just inconsequential journals by people who never do anything exciting. But some are interesting and some are very useful. For example, there is a blog by one of the team working on the new Internet Explorer 7, giving web designers a chance to make any necessary changes to their sites before the browser is officially launched. One blog I regularly visit documents some of the errors and inaccuracies in Germany's biggest mass-circulation daily newspaper.
Incidentally, the very first website was a blog, although the word "blog" didn't exist back then. It was written by Tim Berners-Lee, the man credited with the creation of the world wide web, and announced new servers coming online.