We can organize our content using cateories and subcatgories. A book is a good example, with chapters that separate parts of the book, and subsections of each chapter that separate aprts of a chapter. This type of organization is hierarchical, like an upside down tree.
Another way to organize content is by taxonomy. These are keywords set up by the site owner or librarian. These keywords allow documents to be organized in a non-herarchical manner, allowing for a different search technique.
For example, if we had a fruit and vegetable database, fruit could contain Delicious apples, which are red. We could also have a vegetable category that might contain a tomatoe, which is red. If you wanted to find all red-related fruit and vegetables, a taxonomy would link the apple and the tomoatoe together, to allow this relationship. Therefore a search for related "red" fruits and vegetables would be easy.