From blogger.com

Q: What does "blog" mean?
A: "Blog" is short for Web log.

Q: What is a blog?
A: A blog is similar to an online journal. It is composed of short, frequently updated posts arranged chronologically.

Q: Why use a blog?
A: People use blogs for many reasons. Some provide links to other content or commentary about other web sites. Others use blogs as diaries or as a place to share news, photos, poetry, short essays and project updates.