CMS stands for Content Management System. It's a system for managing digital media and text on the web without having to get your hands dirty. A blog is a good example of a CMS. You log in to the system, an editor pops up, you enter your text, you save it and it appears on your site. Without a CMS, you might have to write your entry offline, save it in a special format, then upload it to the server via FTP and possibly have to edit some code to get your new content to appear.
A CMS makes life so much easier. Content is usually stored in a database so it takes up less room, the system handles all the linking and code writing and, unless you want to change something in the structure, you never have to see its innards.
A CMS is not without its limitations. The user is bound by the rules of the system. For instance, some systems will automatically add new content to your menus while others require you to manually create menu entries. The tradeoff is that, with the latter, you have more control of where and how the entries appear.
With the system I'm currently using at work, you have to create the content and save it. Then you have to create a menu entry under which the content will appear. After that, you have to tell the system how and when to make those menus and menu entries visible.
Once it's setup, of course, your content just drops in but making those decisions can be a little frustrating - especially if you want to emulate how a different section functions and can't remember how you did it... or where.
Yes, I'm complaining - but in a nice way. I'm kinda mellowed out this morning because I'm on some mild muscle relaxers for a neck/back/shoulder spasm and they're relaxing my attitude as well.
I could really go for a nap right now.