European cartidges are named using a standard convention, using the diameter of the bullet and the length of the case, both in millimeters, and occasionally the designer or manufacturer's name. This scheme is generally adequate, although it does not completely describe the cartridge. A 7.92x57mm, then, has a bullet that is 7.92mm in diameter, and a case that is 57mm long. A rimmed cartridge will have an 'R' after the case length, for example the 7.62x54Rmm. Sometimes, though, common usage will abbreviate or otherwise muck up the proper designation. For instance, the 7.92x57mm is sometimes referred to as an 8mm Mauser, since it is primarily used in Mauser pattern rifles.
If you're already confused, perhaps you might want to peruse this primer on cartidge terminology.
Disclaimer: The information provided here is generally accurate, but intentionally omits some technical detail, and numerous exceptions, in the interest of clarity. For a more detailed discussion of metallic cartridges, try any one of a number of reloading manuals. For a more-or-less comprehensive review of current and obsolete cartridges, try the lastest edition of Cartridges Of The World.
22 Long Rifle
22 Winchester Rimfire Magnum
300 Winchester Magnum
45 Automatic Colt Pistol
50 Browning Machine Gun