Categories are how we recognise, differentiate, group and understand different things.

We do this all the time in our daily lives. For instance we categorise foods into what we like and don't like. We categorise things in our in-tray as urgent, not so urgent, do it tomorrow or forget. We categorise experiences as good or bad. 

There are many ways of categorising things.

We can create hierachcical structures for our categories. These are called taxonomies. Or we can create flat single levels of categories - in other words just a list of terms.

The problem with categorisation is that people have different concepts ogf how things should be categorised. For example a child's view of a taxonomy for cars might be Size, Colour, Shape, Speed. An adults might be Fuel economy, Number of Seats, Engine Size and so on.

Another example is personal computers. A consumer categorisation might include How easy is it to setup, Can I connect to the Internet, Size, Price. A computer expert might be Disk size, RAM size, Processor type and so on.

All these category systems are right for their partiular audiences but perhaps wrong for all of them

A well constructed category allows the classification of content in ways that allow the user to access content without having to think too much.

Drupal's categories system is very flexible. It allows all sorts of top-down taxonomic systems that alow solutions to the problems outlined above.

But it also allows user-centric and unstructured systems as well.