Drupal is a framework from which to create and display content.
All content in Drupal is saved as a NODE. A NODE is the most basic type of content, and nodes make up all the content on a Drupal site.
When you create a new peice of content, you are essentially filling up a node with that content.
When Drupal displays the content, all it is doing is accessing that Node, taking the data from that Node, and displaying it on the site. Nodes are the building blocks to a Drupal site.
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Okay, if someone can confirm that all the above is basically correct, then I will have understood the concept behind Drupal.
Now, let me wrap my mind around CCK:
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A NODE can be expanded by using CCK. The purpose of CCK is essentially to add more information to a NODE easily. This allows all the content on a website to easily be expanded in the future without having to associate something with all the previous articles of content, because with CCK, you simply add something to a NODE which is immediately available to all nodes.
If for example I wanted to always attach an image to a story (e.g. to represent that story for a slideshow), using CCK would allow me to add to all Nodes an image which then can be recognized by Drupal, and displayed on a Drupal site.