The Sniper is equipped with a high powered, long-range, high accuracy rifle. Usually snipers take position on high grounds (rooftops, upper store windows, balconies) and rarely change position.In harder difficulties, they deal more damage, have more accuracy, and shoot faster.
Their special ability is the ability of using the "Prone Position", this can make them less noticeable and harder to shoot.
Tactics
In most cases, the best tactics against snipers is good use of cover and use of high accuracy long range weapons such as Assault Rifles and Sniper Rifles.
For Medics and Engineers, that usually lack weapons of good accuracy for long-range battles, pistols are recommended.
Trivia
Like Demoman, Snipers use their pistols to shoot players that are too close.