The Karabiner 98k, Kar98k, was introduced in 1935 as the standard service rifle of the German infantryman. It's predecessor was the Gewehr 98 which was originally invented in 1898. As the Germans realized that a rifle at the length of the Gewehr 98 plus an attached bayonet is a quite long weapon. Therefore they designed a new rifle type. The Gewehr 98k, this was in late 1917. As the war was lost for Imperial Germany just one year later, nobody really cared about a re-armament of the German Army. As the Treaty of Versailles limited the total number of servicemen to 200.000 and the testing grounds for weaponry was very hard to get.
As the Nazis took control of Germany in 1930s and turned it into a dictatorship, the German Army, the Wermacht, needed to be rearmed for the Nazi's war. The Heereswaffenamt gave orders for a new small arms. The Karabiner 98k was brought into service in 1935. It was constructed to chamber the 7.92 x 52mm and it could receive the Seitengewehr 98 bayonet just like the Gewehr 98.
In World War II games like Call of Duty or Medal of Honor the Kar98k is the main armament of the German soldiers. It is a very powerful weapon and is capable of taking out any allied soldier from short to long distance by one shot. It is a fire and forget weapon, cause you don't have to care about the target anymore. The sights differ from game to game.
The Karabiner 98k was the main service rifle of the Wehrmacht.