It is a carbine sized bolt action rifle, adopted as the standard infantry rifle in 1935 by the German Wehrmacht.
The weapon has a capacity of 5 rounds and the player can carry an additional 30 rounds for a total of 35. It's a very powerful single shot bolt action rifle that's highly accurate at long range. It occupies a slot in the lower section of the weapons HUD along with other single shot weapons.
In all three Mafia II DLCs they're available for purchase at Harry's shop from the start.
You can find more real world information about the K98 Rifle on its Wikipedia page.