Quick answer
Both bundle several insulated cores together; the difference is the shape. Round multicore is the general-purpose form for appliances, motors and control wiring. Flat multicore has a slim profile designed mainly for submersible pumps and tight installations, where it sits neatly along a pipe.
See single-core vs multi-core for the basics. APAR offers both in several ranges — round (Mahashakti, Shakti, AlumShakti) and flat (Mahashakti, Shakti, AlumShakti). Browse all multicore cables.