Nomothetic is based on what Kant described as a tendency to generalize, and is typical for the natural sciences. It describes the effort to derive laws that explain types or categories of objective phenomena, in general.
Idiographic is based on what Kant described as a tendency to specify, and is typical for the humanities. It describes the effort to understand the meaning of contingent, unique, and often cultural or subjective phenomena.
In anthropology, idiographic describes the study of a group, seen as an entity, with specific properties that set it apart from other groups. Nomothetic refers to the use of generalization rather than specific properties in the same context.