A half-cab a skateboarding trick that can be best described by telling its origin. It is basically half of a caballerial. That is, it is a fakie backside 180 ollie. This trick is usually done when street skating or otherwise. It can be done on vert also, but is a lot less common there.
Half-cabs are usually done when the skater wants to revert back to going forward again, and rather that just pivoting, wants to add some style. It is a rather smooth way to do so. Also, it is useful for doing into a grind, because that looks pretty cool too. I recommend trying one today, because they are so much damn fun.