What is the main difference between PSLV and GSLV?

Polar satellite launch vehicle (PSLV) is used for launching remote sensing, earth observation satellites etc in low earth orbits. Geosynchronous satellite launch vehicle (GSLV) is used for launching very heavy communication satellites in Geosynchronous transfer orbit (GTO). You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Indian Space Research Organisation [ISRO Notes for UPSC]
2. Mars Orbiter Mission (MOM) – A Complete Overview of Mangalyaan Mission