CMS is just a 'content management system'.
Essentially it provide lots of customisable templates that get their content from a database.
Your create pages by putting various elements (banners, articles, blog entries, categories) where you want them.
Example:
I have a website template with headers etc.
I have a category 'model engineering'
If I create an article on model engineering subject, I assign it to that category.
I decorate the article with images from a 'media library' that i have uploaded.
If I go to the 'model engineering' category they category-specific information appears and it also lists as many of the articles as I want, a few as short excerpts and more as short links, automatically generated.
Click on one of the article excerpts and it then loads the full articles inside the overall template. The article includes a link-back to the categories.
I can also have a page of categories.
I can freely link to pages as well as using this automated system
There are other types of content – banners, ads etc. as well
Everything has sensible defaults.
This makes creating a usable website as simple as populating and basic page template, then adding categories and adding articles. If you don't change the templates this is as simple as typing in text, adding links and uploading images.
Everything appears neat and tidy automatically in the templates and re-organises to fit screen size (even adapting to phones).
If I change a template or style, it automatically 'rolls out' through the whole website, so a major style change only needs to be done once.
If I wanted other people to contribute, I can set permissions so they can add articles but not edit categories or change templates, for example.
This is how a CMS works, it make it very easy to have a consistent feel through your website but also allows a huge number of editors to add content without 'breaking' the design of a website.
Neil