A sniper scope is a customized device that consists of components that place a targeting reticule (crosshairs) over an amplified image.
Snipers compare point of aim to point of impact while sighting a target using a scope. Simply put, where you aim while firing a bullet from more than 600 yards will not be where the bullet lands. During its long journey to the target, the bullet is subjected to a variety of factors. Snipers want that their point of aim and point of impact be the same. Once range, heat, and windage have been considered into the shot, they line up these locations with tiny adjustments to the scope.