CMS : Content Management System

"separates the content of a web site (the text, images, and so on) from the framework of linking pages together and controlling how the pages appear"

A CMS is a system that separates the content of a web site (the text, images, and so on) from the framework of linking pages together and controlling how the pages appear. In most cases, this is done to make a site easier to maintain than would be the case if it was built exclusively out of hard-coded html pages. At least, that is the goal.

How do I pick the best one for me?

First, get a clear picture in mind of what you're going to want to do with it. Spend as much time as you need to make that mental picture as crystal clear as you can. Most CMS offer ways to expand them or to add functions, but you're way better off to start with the right system.
Once you know what you want, try out a lot of them. Look for the one that does what you need it to do with the minimum hassle. Try to find the one that looks and works closest to your ideal. The less customizing work you have to do, the better.
Pay particular attention to the admin interface, since that's where you are going to have to do your work. Make sure you understand what all of the options do.
Check the forums and find out what other users have found out.
Check the way your data will be dependant on that system !

Which type ?

Portals (Yahoo) - Blogs - eCommerce - Groupware - Forums - eLearning (WebCT) - Wiki - Specialized (Galerie)

Try them, avoid too much need for customisation.

NB. : http://www.opensourcecms.com/ is just about PHP/MySQL based ones ...

Admin Interface

How dependant are you from one system ?

What are the backup procedures ?