Hi Raju,

      The basic difference between GMAT and CAT is that CAT is a national level examination and GMAT is a global level examination. CAT scores are conducted by IIMs and its scores are accepted by only colleges in India. Whereas GMAT is held by  Graduate Management Admission Council and its cores are accepted worldwide even by IIMs for NRI candidates in executive programs.

GMAT is for doing MBA in foreign countries and CAT is doing MBA in India .

In GMAT the verbal section is difficult and Quant section is easy GMAT is of 350 marks and CAT is of 300 marks In CAT Quant is Difficult but verbal is difficult . 

You can appear for GMAT anytime in a year but CAT has a fixed date and is nearly in last week of November.