On Board Diagnostics Version 1
On Board Diagnostics Version 2

Basically they're government mandated forms of emission controls integrated into the ECU. All 92-95 Hondas are OBD-1, all 96-up are OBD-2. OBD-1 are more easily modified (i.e. chipping, etc) and as far as their adaptiveness are a bit more forgiving of stuff like advanced timing. The OBD-2 ECUs will figure out you've spun your distributor and will retard the timing enough to get rid of the advance you dialed in which is kind of a pain.

The typical statement you'll hear is "OBD-1 is better for mods than OBD-2 because uh..."