To simplify, a content management system is usually a backend administration area where you can compose and edit the content of your website. If you choose to use a software package such as WordPress or Joomla!, there will be a built-in backend area that comes with the software. They generally include a media manager as well where you can upload photos, images, and files. When dealing with Joomla!, you can even manage your site from the front end.
A website that uses a content management system (CMS) is usually database-driven. There are some instances where you can have a CMS that is file-driven but you won't see that as often. A custom site can also have a custom CMS built for you. In the case where a site would use Twitter's Bootstrap for the frontend, a CMS would have to be built to connect to the database.
The features to look for in a CMS (as mentioned above) is a place to upload pictures and files, and a place to add and edit the actual page content. You will see boxes to fill in your titles, descriptions and keywords for search engine optimization (SEO) and a larger box for your content with an editor. The editor will have some tools on them for you to change fonts, styles, add paragraphs, special characters, weblinks, etc. You do not want to play around with fonts too much as most sites will have stylesheets that dictate the overall fonts, colors and styles for the site. Adding too many extra fonts may make the site look amateurish and busy. We fondly refer to an editor as a WYSIWYG (what you see is what you get) as it will preview what will go on the site.
Another important feature of the WYSIWYG editor are a set of clipboards. If you tend to want to write your content in Word or have emailed something in Outlook and want to paste it in, it is very important to use the little clipboard with the blue W on it (for Word) so you will remove TONS of unwanted code that will hitch a ride when you copy and paste. Most editors have a link called Source or HTML that allows you to see the code generated as you have typed in your content. If there is a lot of gibberish, chances are you pasted without using the little clipboard to remove the extraneous content.
The advantage of a database-driven site with content management is you can easily make updates by logging in and making the changes. You don't have to worry about ftp or making sure you have the latest version of the page. Whichever content management system type you are interested in, total production can help you find the right solution!