A content management system (CMS) is software used for keeping a web site up-to-date. Arguably, the largest benefit of using a CMS is that it provides the ability to reuse content, reducing the number of locations that data must be entered or updated.
The following scenario clarifies this point. Imagine that the phone number for your office changes. Using a CMS may allow you to update the phone number in one location and the change can be propagated throughout your site to update all locations where your phone number appears.