The study of groups.
There are many generalizations and related structures; for example the vertical categorifications of groups like 2-groups, horizontal categorifications as groupoids, groups with structure, like topological groups, Lie groups, thus also Lie groupoids, Lie infinity-groupoids; and noncommutative generalizations like quantum groups. Lie and algebraic group(oid)s have their infinitesimal precursors like formal groups, local Lie groups, tangent Lie algebras, tangent Lie algebroids etc. In the smooth context the relation between Lie groupoids and Lie algebroids is the subject of Lie theory.
Textbook accounts
in relation to applications in physics:
See also:
On aspects of group theory seen inside homotopy theory/$\infty$-group theory: