Blog is short for Weblog which originally meant a personal log on the web. As interest grew people referred to it as We-Blog and now, simply Blog.
As Lee Lefever of Common Craft explains "Blogs are websites that are organized in blog posts." It's an easy, inexpensive way to reach a large group of people. At first it was more of an web diary, personal, shared among a small group of friends. September 11th changed all that. Catherine Seippe discussed this for the American Journalism Review, she wrote "But after September 11, a slew of new or refocused media junkie/political sites reshaped the entire Internet media landscape. Blog now refers to a Web journal that comments on the news--often by criticizing the media and usually in rudely clever tones--with links to stories that back up the commentary with evidence."
Blogs are usually in reverse chronological order, giving readers the feeling of getting the information as it happens. They share a common format so that readers can skim through it quickly. Skimming another necessity in our multi-tasking world. Postings are often short, and to the point. So instead of rambling on about blogs, check out the video from one of my favourite blogs commoncraft.