I can't remember the extact figures but the OBD2 standard defines that the vast majority of fault codes are standard. Just look up "OBD2 fault codes" on the Internet The standard also allows a number of manufacturer specific codes to be defined. If you have a non standard code then it will be a manafacturer specific one which should be easily found with another search using that code.
The fault code gives the identification
First part of the code is either B,C,P or U
B = Body code
C = Chassis
P = Powertrain (Engine & Transmission)
U = Network (Wiring BUS)
the next digit will be either a 0 or 1 - O indicates generic OBD 2 code a 1 indicates vehicle manufacturer specific code.
the next part of the code will be the actual fault code
if it begins with 1 or 2 it is a fuel/air problem
3 is ignition
4 is Aux emission control
5 is speed and idle control
6 is ECU circuit
7 or 8 is transmission
so a P03xx code will be an ignition problem - obviously xx would be a code for a particular circuit.
You will generally get a description of the fault if it's a standard (generic OBD2) code or just the code if it's manufacturer specific.