Got this from a class I was in recently, thought it was interesting.

How to basically read OBDll Standardized Diagnostic Trouble codes.

The codes start with a letter:

The trouble code area:
P- Powertrain
B- Body
C- Chassis
U- Network Fault

Who created this code:
0- SAE defined code
1- Manufacture defined code
2- Manufacture (they ran out of codes for 1 so they expanded to "2")

System:
0- Total system
1- Air-fuel control
2- Air-fuel control
3- Ignition system or misfire
4- Auxiliary Emission Controls
5- Idle or speed controls
6- Computer system (PCM or I/O)
7- Transmission
8- Non-computer controlled Powertrain

The last two digits indicate the specific problem area

So the code P1711 would tell us:

P- Powertrain
1- It's a manufacture's code
7- Transmission
11- Transmission oil temperature circuit out of range

and the code P0300 would tell us:

P- Powertrain
O- It's a SAE mandated code
3- Ignition system or misfire
00- random misfire detected