When taxonomies are arranged hierarchically, it means that successive categories, ranks, grades, or interrelated levels are being used. In a hierarchy, a user would have to select a labeled grouping to find things within it. A hierarchy of movies might look like this:
Hierarchies tend to follow two patterns. First, a broad and shallow hierarchy gives the user more choices up front so they can get to everything in a few steps. As an example, in a grocery store, you choose an aisle, and each aisle has certain arrangement of products, but that’s as deep as you can go.
A narrow and deep hierarchy gives the user fewer choices at once. On a large website, like usa.gov, a few high level options point users to more specific items with each click.
When individual pieces exist on one level without further categorization, the taxonomy is heterarchical. For example, each lettered box in the arrangement in this illustration is heterarchical.