A content management system (CMS) is a computer software system for organizing and facilitating collaborative creation of documents and other content. A content management system is frequently a web application used for managing websites and web content, though in many cases, content management systems require special client software for editing and constructing articles. The market for content management systems remains fragmented, with many open-source and proprietary solutions available. (Wikipedia)

Imagine if you could go to your website, type in a username and password, and update anything that you wanted  in a MS word type interface.Well, that is basically what a CMS does. It allows you to make all sorts of content changes to your existing website.Not only that, but you can also have other 'content managers' help you update you website by giving them access to your website. Most CMS allow you to administrate users and give them roles ...editor, manager, administrator, etc. Your can also have regular users if you are creating member-based site.

CMS USES

Here are some examples of what typical website business owner might want to update ...
News & Events
Latest Products/Services
Contacts Updates
Clients Showcase
Portfolio
Case Studies
White Papers
Articles

CMS FUNCTIONS

With CMS you can create different types of function on your website ...
As a product catalog
Document Management System/Document Tracking
Corporate Intranet
Real Estate portals
Job Postings
E-Commerce Applications

CMS FEATURES

These are some typical CMS features that can be very useful ...
Easy handling of users according to roles, eg admin role, editor role, viewer role etc.
Easy update of website content using backend forms.

As you can see, a CMS system is a very powerful tool when you are trying to succeed with your online presence. It also saves you a lot of time when you need to update you content. CMS costs a little more than your typical website. But, it will save you thousands in the long run as you can edit the content yourself.