There are two ways to describe a CMS and both have their uses. The first is a technical/academic description and the second is a plain-English description. For a technical/academic description, see this Wikipedia article.

Now, on to plain-English. For most nonprofits, just one type of CMS is of interest, namely a web-based CMS. So what does it do? Well it's a system for managing content. But what is content? Well think of any web page you look at. If it is a news web page then its content is likely news stories that change often. If it's a blog its content may be updated only occasionally and will be personal stories, pithy advice, tirades, etc.  For an email site the content would be the emails themselves. For a store the content will be the pictures and information about products and services they provide. In short, anything you see on a web page is content.

So why do I need a management system for this content? Well, if you create a website manually you will have to have some knowledge of technical tools like HTML and CSS, or you'd have to use software like DreamWeaver, iWeb, FrontPage, etc. At that point only those who know how to use the software can change the webpage and the structure of the page may become cumbersome and unwieldy. Further the content will be limited to what is typed. It's generally awkward, but still manageable. But you need a better way. Arguably the best way is to make use of a CMS.

So what would a CMS give you? The short version is it will give anyone who can use a web page the ability to control/adjust the content, usually just by signing in and then clicking an edit botton/tab from the page you want to change. The long version? Well lets imagine your organization wants to have a web page that has the following content:

About Our Organization
Latest News
Photos of Our Projects
Our Company Blog (maybe several blogs)
How to Contact Us

How does a CMS help? Well a CMS can give you all of this out of the box with relatively little effort if you have some experience, or with some effort and perseverance if you have a staff member who can figure their way through it. A typical scenario is where a dedicated volunteer or hired web developer will set up your new web page for you and then your staff will be the ones who then maintain the content on the page, and they will do so through simple forms and menus. They will not need to have special skills beyond those that any web surfer/normal web user already has.

And what will your developer do with the CMS? The CMS will always include membership/sign-on control, so you could give sign-ons to your website to staff, and also to customers if you choose. You can give each person a role and then specify that some content can only be created/updated/viewed by certain roles. For example you could say that only a staff member can type a blog entry, but any registered user (subscriber/supporter) can comment on a blog entry.

Blogging tools are also usually built in to a CMS, along with all of the associated controls, for example having someone approve all comments before they become publicly visible.  A CMS will also include the ability to add other basic content, and this will easily let you handle your About page and also to specify who can change it (you might let every staff member have a blog, but then only let one or two of them add news or edit the About page.

You can use a blog to handle your News page, or you could use some other simple content creation tool. If you want something a bit more advanced then CMS generally have add-on tools that can be added to your CMS to add functionality. These tools usually include the ability to create complex collections of information and display them in complex ways. For example your news could have a date associated with it and also some taxonomy/tag (e.g. you might tag news as "financial", "drive", "event", etc. Then you could decide to control how your news displayed by allowing visitors to choose what kind of news they want to see. You could also automatically drop news items older than three months.

CMS usually include photo galleries, or you could use advanced tools to create your own gallery. Finally, Contact forms are also generally built right in. Throw in the ability to have several versions of the same content and the ability to revert back to a previous version of a page should a bad edit be made, along with the ability to have multilingual sites, and to pull RSS feeds into the page, and to have full-blown forums, and more, and you have a tool that will not just get you to this first step, but can take you well beyond it to a far more complex and rich site, possibly even one that includes Constituent or Customer Relationship Management (CRM). In short, a CMS will give most nonprofits all the web power they need and more.

It's a horrible oversimplification, but in broad terms an experienced developer could put a simple site like this together in a few days, more to do it right, still more if it's complex and yet more if you want it to look pretty. It's important to get it right though as you will have to live with the results or get the developer/s back to tweak it.