To add more,
It depends on which flow you are referring to.
Anything to do with the logic, algorithms etc comes in frontend.
If you are designing a Digital Design, Then RTL coding, Verification forms the front End.(Ex MicroProcessor). Anything that refers to the Physical design of the chip,
Floorplanning, Place& Route, Clock tree Synthesis, Layout, DRC, LVS, Parasitic Extraction evrything comes is usually called as backend.
If you are doing analog/RF/DSP Designs, transitor level Schematic design and Simulation, Matlab etc forms your frontend. (Ex, OpAmp, ADCs etc)
Custom layout, DRC, LVS, Extracion, DFM everything becomes a part of back end.
If u r new, Think which one is of your interest and decide accordinly. and enhance your skills slowly in that.