Rapid Fire is a hunter ability that gives the hunter increased ranged attack speed. It also reduces the "casting" time of [Steady Shot]. By allowing you to shoot faster, it significantly increases the damage you can do for a short period of time, and effective use of the ability is central to hunter DPS in groups.
Improvement
[Posthaste] reduces the cooldown by 1 minute per rank, down to 3 minutes. The Striker's Garb 5-piece set bonus also reduces the cooldown to 3 minutes. These effects do not stack.
[Rapid Recuperation] causes you to regain 6/12 Focus every 3 seconds and gain 25/50 focus when Rapid Fire.
Tips and tactics
With two points in [Posthaste], Rapid Fire's cooldown is reduced to that of [Readiness]. This means that you can use Rapid Fire, then use Readiness to reset the cooldown, then use Rapid Fire again immediately after effect wears off. Doing this will allow you to use Rapid Fire twice in a row, for a total of 30 seconds of increased attack speed every three minutes.
Using [Bestial Wrath], [Call of the Wild] and any trinkets or other effects which increase your damage, attack power, or critical strike chance together with Rapid Fire will allow you to shoot faster while dealing more damage with each shot. This will result in higher total damage done than if you had used these effects separately.