CCK: Various Data Types (for definition)
Taxonomy: (for classification) allows for organic growth of content
How do you know which to use?
* Are you listing attributes? (color of a pen, type of a car)
* Do you need to store data other than plain text? (Taxonomy only allows plain text)
* How much control do you want over the theming? (CCK allows precise theming)
* Do you need a hierarchy? (Use Taxonomy)
* Should users be able to "free tag" content? (Where community is driving content)
* Do your objects exist in the real world?
Both! Let the features of your site decide.