A bowler is classified according to the speed he bowls in miles per hour.
Bowler------------------mph----------------------kph
***************************************
Fast---------------------90+----------------------144.8+
Fast-medium---------80-89---------------------128.74+
Medium-Fast---------70-79---------------------112.65+
Medium----------------60-69--------------------96.56+
Slow------------------Below 59----------------Below 95
The Fast category is classified into Express Fast (145-150k), Super Fast(150-160k) & Lightning Fast(160+k) .
Ofcourse, there may be other classifications. This one I have posted is based on MCC Masterclass 1996 edition. It is a coaching book. Also, I found the same classification in David Whiley's Art & Technique of Umpiring, edition 1976.
The fact is that the ICC does not have its own classification table. Atleast, it didn't when these books were published. That's how it is mentioned in the MCC Masterclass.