The SKS is a Soviet Era Semi-automatic Rifle. It was developed when the Soviets realised that rifles such as the Mosin Nagant were too long and unwieldy and the ranges that the Mosin could reach were rarely being seen during combat.
This led them to develop an "intermediate cartridge" and a new gun to shoot it. The rifle went into service in 1949, where it almost immediatly got replaced by the AK47, Though the rifle did see active service with other soviet aligned countries for many years afterwards.