I'll bite. Planer boards react to the water like a sail reacts to the wind.
Standard boards stay tracking all the time off the end of the cable and the releases clip to your line and slide out on the cable. When the release pops your line is free from the rig.
In-line planer boards stay attached to the line of your rod at one point and a release keeps it tracking outwards. Once the release pops, the board straightens out and doesn't pull to the side any longer which allows you to bring it in easier and land the fish.
I should also tell you that I don't use them because it limits your ability to maneuver somewhat and power-boats like to run them over.