This tag should be used for any question that's about the Sniper class from Team Fortress 2, Valve's free-to-play team-based multiplier shooter.
The Sniper in TF2 is exactly what it says on the tin: shoot people from a long way away in the head and they die. With the ability to take out any enemy in one shot from extreme distance, the Sniper is a highly skill-based class that many teams appreciate having but will always complain about when losing.
The Sniper's primary weapon is a rifle that, when scoped, always scores critical hits upon putting a bullet into an opposing player's cranium. As a Sniper remains with his eye to his scope, his next shot charges up in strength to the point where a headshot will deal enough damage to kill any enemy regardless of their current health. Uncharged headshots are still strong enough to kill non-combat classes and Scouts, while a fully-charged bodyshot will do the same. The Sniper also carries a large knife and either an SMG or a jar of suspicious yellow fluid for dealing with enemies too close for comfort, and may choose to swap out his rifle for a bow that shoots slightly weaker arrows with notoriously large hitboxes.
A Sniper's job is simple: Assassinate key targets and lock down long sightlines. Killing a Medic, opposing Sniper, Engineer repairing his buildings, or fully-overhealed Heavy in one shot will bring a sigh of relief to the team as a tough obstacle is removed. Once opposing Snipers are removed then a Sniper can simply camp at one end of a long sightline and shoot whatever shows up, creating a corridor of doom for the enemy team. But Snipers are a rather polarizing class. A Sniper who is spending too much time charging shots is not aware of their surroundings and is easy pickings for a Spy or other enemy from an unexpected direction. Snipers are also a popular class to overuse despite becoming less useful the more of them there are; a team with many Snipers has little front-line support and will not win without a large skill differential.