Blog is short for Web log.  A log is like a professional diary or journal.  Remember on Star Trek when the voice over would say Captain's Log Star Date....and then the Captain would fill us in on important information?  The idea was that in the future a person's diary would be kept on a computer with sound and picture instead of written in a book.  Welcome to the future.  Although most people are still choosing to write their blogs a large number of people are already choosing to video tape their blogs.

In any case a blog is diary or journal.

Because blogging is relatively new there really aren't any rules.  You can blog about your day or you can write poems and post them to your blog.  Your blog can be about a very specific topic (such as a book or favorite television show) or it can be just rantings and musings on your everyday life, it can even be about a specific product or company.

Blogs can be written by individuals or groups.  A blog is meant to be written on a consistent basis over a period of time, so perhaps the only true guideline is that your blog be about a subject that you are passionate about.  What do you have to say to the world?