On a most basic level it is a way for you, the site owner, to make changes to your site any time you’d like.  It is a system which allows you to create and edit content using a tool which is similar to working with a Word document.

On a more complex level, it allows you to assign different capabilities to different people on your team – some might be able to write content, but not publish it.  Others might be able to upload image files but not video files and so on.

If you have a site where the information changes fairly often, daily, once a week and even once a month, it might be worth the extra expense to have content management system built for you.

And there are lots of choices out there from custom builds to pre-built templates to open-source “free” software like WordPress, Joomla, and Drupal.

It really depends on what you want.  If you don’t really need much more than the ability to change out text once in a while, then having a simple custom build it probably appropriate.  On the other hand, if you want something with lots more functionality then going with one of the “free” open-source programs is probably the way to go.  I put “free” in quotes because the functionality each system offers is free, but you will still need to pay a developer to do the design customization which makes any site uniquely your own.

For me, web accessibility is very important and its important that I give my clients a system that is fairly straightforward and easy to work with.  My favorite open-source CMS at the moment is definitely WordPress.  Its easy to teach clients to use, it has lots and lots of plug-ins to choose from, and it is very search engine friendly (in my experience it a system is search engine friendly it usually has good web accessibility and usability built in.)

• W 3 Schools: when i was first learning basic html I turned to them often.  I love that they let you test what they are teaching – really helped me understand more about what I was doing.  And I still go there when I need a basic understanding of a particular technology.
• Site Point:  I have several of their books and visit thier forums fairly often.  Lots of good solid information.
• HTML Dog:  When I was working for a large Fortune 500 company I became interested in web accessibility.  At the time, our company was using a navigation system that required mouse clicks (you can’t see the mouse trail if you are visually impaired) and I wanted desperately to show my collegues that there was a better more accessible way to build navigation that still had the cool drop-down action.  I, and many others, are in debt to the developers at HTML Dog for  their “Sons of Suckerfish” tutorial
• Accessify:  I’ve only recently discovered this site, but find some the tools very helpful.
• Listomatic:  If you want some examples and code for quick navigation to here.
• Dynamic Drive CSS Library:  Great starting place for tableless builds