We could say that term groups resemble WordPress custom taxonomies, but there are lots of differences, and term groups are more flexible (and complex).
Term groups are hierarchical and allow for multiple parentage. This means that any term group can be parent or children of any number of other term groups.
A term can belong to any number of term groups. So, there is a many-to-many relationship.
Term groups may relate to terms in another way, setting the term as index of a term group. This means that the term will act as representation of the term group: the link to the term group will be that of the term.
Term groups may have term group checktables. These are meta boxes with the terms of the term group (including or not those of the children term groups), with many features of their own (Checktables can use thumbnails if available, terms may be hidden for a given checktable —rendering them unselectable—, may have preselected terms, have different hooks, may be read only or displayed only if certain term is selected, &c.)