Mate check your Ops manuals for the 'ins' and 'outs' but in a nut shell and very basically put:
Cat A = straight forward aerodrome like LGW etc
Cat B = could have terrain obstacles, funny approaches, odd procedures etc like TFS. The flight crew will most likely have to read a brief prior to operating in there.
Cat C = neither A or B, these are the real fun airports like GIB, INN and CMF. You know, the kind of airfield that will give you a near death experience (joke!.........?). The crew will normally have to have a Sim check for this, Cpts only landing etc.
The categorisation process should be detailed in your Ops manual (like Circle to Land details etc etc) - each AO does its own categorisations and should have it all detailed for CAA inspections etc
Take this as a laymans explanation!