There are basically 6 corners. TT,SS,SF.FS,FF and MC where 1st letter stands for NMOS while 2nd for PMOS. So FF corner means FAST NMOS and FAST PMOS. Similarly TT stands for typical and MC for Mismatch or Monte Carlo. Now by Fast means Less Vth, Less Delay and more Idsat as compared to TT > SS.
For more details use google.