Topic Hierarchy
Topic hierarchies are built in a very similar manner to the science of taxonomy. Though taxonomy is mostly thought of as the biological classifications and subclassifications of living entities, the principals can be applied to any large data set. The main outcome is the ability to have a large berth of data and the ability to simply and logically drill down to the exact element for which you have an interest. This is achieved through taxonomy rank, where a species is described through a series of increasingly specific set of levels or ranks.