First some terminology: “blog” is a shortened version of “weblog” which is a frequently updated personal website. Most blogs have date stamps on the entries, and consist of links and commentary.
Now, remember when you were in school and you would bowl for charity? And for every pin you knocked down you got, say, ten cents? Or run for a dollar a mile? During the Blogathon, people update their websites every 30 minutes for 24 hours straight. For this, they collect sponsorships. Pledges can be a flat donation, or a certain amount for every hour the blogger manages to stay awake.