William Safire writes about the etymology of the word "blog" in this article

Blog is a shortening of Web log. It is a Web site belonging to some average but opinionated Joe or Josie who keeps what used to be called a ''commonplace book'' -- a collection of clippings, musings and other things like journal entries that strike one's fancy or titillate one's curiosity. What makes this online daybook different from the commonplace book is that this form of personal noodling or diary-writing is on the Internet, with links that take the reader around the world in pursuit of more about a topic.

Here's my definition:

Weblogs are personal, online publications that can include links to news stories, opinion pieces and commentary, used for everything from personal journals to intercompany notes to supplementary news on major media sites

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