Spin drift is caused by the spin the of a bullet or projectile. The surface of a spinning bullet spins the air around it. Because the air can not continuously spin around the bullet in a constant motion a low pressure point evolves in the direction of the spin. The bullet by way of higher pressure must move in the direction of the low pressure. If a bullet or shell has right hand rifling then it will drift to the right. The same is true for left hand rifling.
Typically spin drift starts to be observed at about 600 yd (550 m) The tighter the rifling (1 in 7 as opposed to 1 in 13) the greater the drift. This is actually the same effect as the curve ball in baseball. On a baseball, the stitching sticks out and 'grips' the air. The bullet has been deformed a bit by the rifling which results in the same thing. It is also called the Bernoulli Effect, and is the reason airplanes fly. It is the difference in air speed felt by the top and bottom of the wing, due to wing shape, which results in a pressure difference, causing a force perpendicular to the direction of motion. In the airplane the force is up, in the baseball it is to the left or right. Don't forget the spin is needed to prevent unstable motion: tumbling in a bullet, a knuckleball in baseball.