A half-cab is basically a f/s 180 while riding fakie. (If your regular, your right foot should be on the NOSE (the front) of the board, and your left should be in the middle.) Pop a fakie ollie, move your leading foot to the toe side and your trailing foot to the heel side. Your board should follow you. Once you finish the 180 smack the board down and ride away. Some people (including myself at first) feel kind of awkward about this trick since your back is facing the way your going. Don't worry about this. Half-Cabs are actually a lot easier to spin the regualr 180s. And ever though you might fall off at first, wait, till you start doing 180-half cab-180 sequence and you'll feel like you accomplished something.
Tips: This move is easier to do moving, although you will fall off the first few times. Just learn how to balance it. Try spinning the board with your whole body, and mainly your shoulders.

Trick contributed by: www.skateboarduk.com