Not a dumb question at all- but a VERY large field of endeavor. Designations of cartridges is a study in change, in marketing, personal vanity, and multi-nationalism.
There are several good references that cover MOST of the cartridges out there- such as Frank Barnes Cartridges of the World (if you are not a cartridge collector, see if the library can snag you a copy )
I have about a thousand cartridges in my collection, and that only brushes the surface. This, with it's links, will get you started- Click HERE: http://en.wikipedia.org/wiki/Cartridge_(firearms
America and Britain used the inch system- so a .44 fires a bullet ABOUT 44/100ths of an inch in diameter. Europeans used the metric system- instead of a .32, they have a 7.65 mm round.
Some of the names are marketing and copyright- since Colt invented the .45 Auto as it is know known, they named the cartridge the .45 Automatic Colt Pistol. Smith and Wesson invented the .40 Smith & Wesson so you see .40 S&W on guns not made by them- THEY own the name.
Some names, like 45-70, were the caliber (45) and number of grains of black powder (70) The 30-06 is a .30 cal rifle cartridge, created in 1906. The 7.62x39 is a 7.62 mm bullet in a 39mm case- but Europeans and Americans measure bore diameter differently. 9mm Luger is PROPERLY called 9mm Parabellum, but could be called other names as well. And there are a BUNCH of "9mm" rounds that are different sizes, for different guns.
This is an extensive field of study on its own. Do as much reading as you can, and get back to us with questions. We won't bite- really.