Pari-Mutuel

A pool style wagering system, where all money input through betting is divided among those with winning tickets (save for various deductions by the track operators).

Scratch

A horse is a scratch when injury or other necessity causes it to be withdrawn from the race.

Rabbit

Also known as a pacemaker; this is a horse with no expected chance of winning but is used to prompt a quick pace on the front end of the race to the benefit of a stablemate who is a closer.