When organizing information it is common to use either a hierarchy or tags. An element in a hierarchy is ordered within a predefined category and relates to other elements as above, below or at the same level. The only direct link between elements in an hierarchy is its immediate superior and the elements subordinates.
Tags on the other hand are non-hierarchical keywords assigned to an element. There are no links between tags, and elements can have tags on different hierarchical level which in a strict hierarchy would not be possible.
Tag-hierarchy refers to tags who are placed in some form of hierarchy, not necessary in one hierarchy alone.
Questions having the tag-hierarchy tag regards tags in hierachies.