WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. What a mouthful. WordPress is both free and priceless at the same time.

More simply, WordPress is what you use when you want to work with your blogging software, not fight it.  (from wordpress.com)

It can be used for blogging or just a for a real simple website. 

• It's free
• No programming knowledge needed -- just fill in the blanks
• Web based -- update your site from anywhere
• Add plugins for extended functionality
• Have multiple users to allow different people to update the site

• Difficult to change the look of your site -- changes need to be made in template code
• Not practical for sites that need added functionalities such as calendars