Opto-coupler is, as far as I know :roll: , consisting of a photodiode and a phototransistor enclosed in a small DIL package (like 555 timer IC). Transistor acts as a switch and diode acts as an activator. When you apply voltage to the diode, the transitor lets the current flow through it. Otherwise, current flow is blocked.
Optocouplers are mostly used when you need to completely isolate one part of a circut from another one, but still be able to controll it (i.e.: if you want to switch 220V AC with your computer using the parallel port). There are different types of opto-couplers available (reflexive, optointerrupters) and with different output (standard transistor output, schmitt trigger, darlington output, etc.)
Hope this helps. I'm not an electronics wiz, so don't take this for granted.
P.S. I'm using reflexive opto-couplers for IR proximity detector for my robot.