Bottle necked, Rimmed .30 caliber rifle cartridge
.30 is the (approximate) diameter of the rifle bore for the caliber cited (in inches). -30 refers to the smokeless powder charge used in the cartridge as originally designed.
A Winchester design originally designated the .30 WCF (Winchester Center Fire) and still sometimes referred to in that way, the .30-30 was designed in 1895 specifically for use in lever action rifles. Marlin chambered their competing lever action rifle in the caliber a few years later but in order to keep from using their competitor’s name, redesignated the cartridge .30-30 for their purposes. The new name stuck in popular use, although it is often referred to as the .30-30 Winchester.
The .30-30 fires a bullet between 100 and 175 grains at velocities of up to 2600 feet per second for a muzzle energy of up to just over 1900 lb/ft.
As a load intended for use in lever action rifles, the .30-30 uses low ballistic coefficient bullets and so its range is limited, but is well known as an excellent hunting arm for wooded or brushy areas.