Josh Lowensohn who wrote the guide for Webware kicks-off:

What is Facebook and why should you use it?

Facebook is a social networking service that lets you connect with friends, co-workers, and others who share similar interests or who have common backgrounds. Many use it as a way to stay in touch after finishing school, or as a way to share their life publicly. What makes Facebook different from other social networks are its extensive privacy controls, its development platform, and its large and quickly growing user base. Facebook has been called the “thinking person’s” social network. Compared to many other social networks, Facebook gets new features and improvements on a regular basis.

Don’t get too comfortable. “Facebook gets new features and improvements on a regular basis” means don’t blink or you’ll log on one day and not know what to do or how to do it!

Just in case you’re hungry for more, from Robin Good there is Social Networking: A Beginner’s Guide To Facebook, another handy reference with some additional stuff to work through.