The main differences between categories and tags, as I see them are:

- Categories are usually a predefined where tags are more user generated and flexible.
- Categories tend to be displayed in a tree structure where tags are normally displayed in a flat structure.
- Categories normally don’t include a visual display that reflects popularity. Tags on the other hands are normally displayed in a buzz cloud that reflects the tag’s popularity.

The question is what is more effective / friendly the simple predefined set of categories or the limitless, flexible tags?