They're called whatever the manufacture wanted to sell them as.

Take the .38 and .357 both are the same size but neither is actually .38 or .357. The 9mm is also a ".357 round"

[b]Generally[/b], military rounds are metric, i.e. 5.56x45mm vs. .223, 7.62x51mm vs. .308, 9x19mm vs. .380 long. [:\]

Again, that's just a generalization and certainly not universal.

This is not a "dumb" question, but it is info you should be halfway knowledgeable about eventually.  The European system of cartridge designation tends to be the bore diameter X case length, with an "R" at the end if rimmed.  Therefore, our ".308 Winchester" becomes "7.62 X 51 mm."  Our .223 Remington becomes 5.56 X 45 mm.  What we just call 8mm Mauser is really 8 X 57JS mm.  The "J" and "S" mean other things relating to changes made in military cartridges.  All in all, the whole system is not consistent and names are mostly just names given by the designer/marketer.  Another good example is the "44 magnum"--bore diameter is really .429"--not a 44, but really a 42.9, but that doesn't sound so good.  A .45 is really .451, so it is real close.  A 9 MM measures .355", a .357 and .38 are really .357".  Our various thirty caliber bores measure .308", but the British .303 Enfield measures .311", even though they are on the metric system.  The Russian thirty caliber (7.62 X 39 mm and 7.62 X 54R mm) measure .311".  To further complicate things, some measure across the grooves, and some measure across the lands (rifling).  Nothing makes sense, there are literally hundreds of examples, some are just for marketing, anything goes, so hang on, and enjoy the ride!