Melee

Melee (pronounced Mell-ay in British English) is a form of attack which shunts back the target rendering them stunned for a small amount of time (1-3 seconds, depending on infected type). It is extremely useful in defense against large hordes of Common Infected and rescuing other Survivors caught by Hunters or Smokers.

Rescuing other Survivors
When another Survivor is pinned by a Hunter or snared by a Smoker, the best and quickest way to help them out is to melee them, which will free them instantly.

Hunter
A Hunter will quickly tear away at a Survivor's health, so if you're low on ammo, or reloading, it can be best to melee the Hunter off to save them. Once a Hunter is knocked off, they will be stunned temporarily, and after this time can sometimes make an escape to set up another ambush, or they could pounce immediately after.

Smoker
A Smoker's drag can be interrupted with a quick swipe of melee, rendering it harmless for another 15 seconds. This means an effective team can save a Survivor from taking any damage if they quick enough reactions (though in Versus the Smoker will damage as it drags). Some people make the mistake of trying to kill the Smoker instead of freeing the Survivor, but this wastes time and health, as it is quicker to free the Survivor than to kill the Smoker. You can also melee the Smoker to free the Survivor. A Smoker will be stunned for about 1 second after their tongue breaks, meaning you have some time to deal damage to it before it retreats.