The theory of homological knot invariants - the categorification of polynomial knot invariants - appeared 15 years ago, and has been very active ever since. As in the case of the the quantum polynomial knot invariants, they turned out to be related with numerous different fields of mathematics (including topology, quantum groups, representation theory, homological algebra, von Neumann algebras, etc.). In this talk I'll present a basic overview of this categorification in the case of the HOMFLY-PT invariants - both concerning their definition and their properties. Finally, a particular recent application will be shown related to the physics interpretation via BPS invariants, which implies some surprising integrality properties of a pure number theoretical interest.