CCK is the Content Construction Kit.
The main idea is to make content management more flexible than simple Joomla content manager. To give you ability to add another fields to article submission form apart from HTML input.
For example you want download or file manager. Apart from HTML file description you want to upload file. You add files field that allow you to upload file on article submission.
Fields are later used not only to display additional information information but also to filter or search though. Let's say you have a car catalog. And you added fields Year and Model to the article submission form. not only now you can see this in the article/listing text but you will be able too look/search for car between 2004-2005 only BMW-X7 series.
Still not clear?
Then you may consider CCK as a form builder. Actually it is nothing more than that. It is form constructor. You create form and add fields to that form like select, checkbox, text, … Then you can use this form to get data from this form. Previously form data was send to email. But imagine we store it in DB. And we created nice view to list those entries. And we added a lot of feature to operate with those entries like it would be an article. You can publish, unpublish, hide, bookmark, watch, …
As addition you can read nice explanation about what are types and section/categories here.
That is it.
Please watch also Cobalt quick start tutorial to get more ideas.