a CMS is a Content Management System. There are several different types…the ones that are easy to edit – basically your site with an admin login, have editing tools directly on the site and it makes it easy to see what your editing.
There are others that are more complex, but make big, intense websites easier to handle – they have a separate login as well but the information is organized more into folders (or whatever way you decide) versus looking like your webpage.

I actually prefer not using a CMS. It allows your design to be completely flexible, as no design template system with a new learning curve will get in your way.

I usually create a text file with content HTML for each intended web page and have the .htaccess file forward requests to a template.php script, which automatically injects the template around the text. This method has proven to be very simple and functional for me, so I encourage you to give it a try.

If I absolutely need a CMS, I will design my own minimalistic, easy-to-use CMS with features specific to the application I’m designing.

A Content Management System is essentially a work flow process for content. Think of it as a conveyor belt for ideas and written communications. With all messaging there needs to be collaboration and approvals. There are schedules to meet, and the versioning of the same messaging needs to appear in several mediums. Not only does a CMS facilitate the creation, scheduling and tracking, but it also allows for the storage of the final message, and all the parts – graphics, content, approvals, talent agreements, costs, etc.

With a web site, you would start with a wireframe of the site, and track content to the areas of the site where it belongs. If you needed the same content and graphics to be versioned for print, you would be able to do that.

Basically a CMS is a set of pre-built set of templates for building sites. There are templates that control the page layout, the headers, and navigations… as well as templates that control articles, image galleries, forms, and just about anything else you want for a page. These templates allow a user to build a custon website with limited or no knowledge of html, javascript, PHP, or other languages.
You have hundreds of options for CMS products. A good place to compare features is CMSMatrix.