The Demon Hunter gains 4% Critical Hit Chance every second, up to the cap of 100%. The bonus is lost 1 second after scoring a successful critical hit, however.
Sharpshooter is a Demon Hunter passive skill, unlocked at Level 50.
This ability is one of the few legitimate means to get a 100% Critical Hit Chance. However, in game it is less powerful than it sounds: as soon as the player's own Critical Hit chance becomes high enough to score at least one Critical Hit per second with either of the skills, Sharpshooter effectively cannot give more than 4% Critical Hit chance at a time. Its main advantage is having a 100% chance to critically hit all enemies reached with an area of effect attack at the very beginning of combat, in most cases wiping a whole bunch of lesser enemies before they even notice the character (it helps to have the Ambush skill as well). Because of this, Sharpshooter proves to be most effective when combined with powerful attacks like Cluster Arrow or Rain of Vengeance.