A hierarchic data model organizes in the form of a hierarchy or tree structure. Early databases and programming data structures commonly used hierarchic models, but these fell out of favor. In the database world the RelationalDataModel became dominant, while for most in-memory programming the NetworkDataModel dominates. This was due to the fact that a hierarchy, while a simple organizational tool, breaks down as you get more complex data.
Yet the simplicity of a hierarchy is hard to deny, and hierarchic models keep showing value. The big return of hierarchic data models these days is with the rise of XML, which with its various associated technologies offers standard facilities for manipulating hierarchic data which are similar to what SQL offers for the RelationalDataModel.