Sabre is the only ‘cutting’ weapon in modern fencing - hits can be scored with the edge of the blade as well as the tip. The valid target area is the head, arms and body above the hips. If a hit lands off target, it is ignored (unlike foil, where an off-target hit stops the action).

Sabre is a 'priority' weapon. The first person to start the attack has priority until they miss, or are parried, or stop attacking. Using parries, footwork and tactics, first to attack does not mean first to score a point. 

The blade is 88cm long, and the guard curves over on one side and protects the fingers during parries. Sabre is the lightest weapon, and is manipulated with the fingers for speed. The cutting action means the tip can move towards the target considerably faster than for foil or epee.

With a large target area to be defended from both thrusting and cutting attacks, sabre bouts have fast footwork and blade work as each fencer manoeuvres to create an opportunity to score a hit while denying their opponent such an opening. Competition bouts are very intense but of shorter duration than for the other weapons. 

Speed is more important than long arms, but as for all three weapons, technique and tactics are far more important.

In the late 1800’s, a heavier weapon, similar to a cavalry sabre, was used. This required heavily padded jackets, gauntlets and a very strong mask. The current weapon was first used in the early 1900’s and was almost universal by the early 1920’s because it allowed much faster, more dynamic fencing and did not require heavy padding. 

For more information, see the Wikipedia entry on Sabre.