A bit is used by horsemen to communicate with their horse while riding.
It rests inside a horse's mouth in the back on the gums where there are no teeth and is held in place with a bridle.
A couple of common designs include:

• Snaffle bit - consists of the bit mouthpiece with rings attached on each side and works by applying direct pressure without leverage on the mouth.
  It applies pressure on the back gums, tongue and corner of the mouth.
  
• Curb bit - uses a type of lever called a shank placing pressure on the back gums, tongue, and roof of the mouth.
  The rider's signal is also felt over the horse's poll and on the chin though use of a curb strap.
  

Bits consists of two basic components, the bit mouthpiece, which is placed inside the horse's mouth, and the bit rings (for a snaffle bit) or shanks (for a curb bit) where the bridle and reins attach.
Bits act with pressure and leverage.
The mouth piece does not define the bit type.
It is the rings or leverage on the side pieces used to act on the horse's mouth that determines if it is a curb or snaffle bit.
Snaffle bits apply pressure directly to the horse's mouth through the reins attached to the rings on each side of the bit.
The rider applies pressure indirectly with curb bits through leverage action of the reins attached to the end of the shanks (levers).
Curb bits place more leverage on the horse than snaffle bits.
The amount of pressure asserted through the reins will be exerted several times greater on the horse's mouth.
When curb bits are used, ride with a loose rein and only use light pressure.
Curb bits are more appropriate for well trained horses.
Snaffle bits are gentler on a horse's mouth and tend to be more mild and comfortable for the horse.
Unlike the curb bit that exerts a much greater force through leverage, the force you apply on a snaffle bit through the reins is the only amount of pressure put on the horse's mouth.