I think frontend deals with highlevel description of circuitry using HDL while backend deals with actual physical layout of the system. Correct me if I am wrong.
VLSI chiefly comprises of front end design and back end design.
While front end design includes digital design using HDL, design verification through simulation and other verification techniques, the design from gates and design for testability.
Back-end design comprises of complementary metal-oxide semiconductor (CMOS) design and its characterization.