Curb bits are, generally, any bits that make use of leverage (i.e. have a lever action). Technically, pelham bits are curb bits, but I consider pelham bits a separate type of bit since they are intended to have the actions of both a curb and a snaffle.
The mouthpiece of a curb bit determines, to a reasonable degree, where and how much (along with the lengths of the shanks) pressure will be applied. With a simple rubber bar used in curb action, pressure will be mostly applied to lower jaw through the tongue and lower teeth (since most human ponies have teeth instead of bars).
A jointed bit will apply more pressure on the pony's teeth (in a bio-equine, this pressure would be applied to the bars).