OK here's a quick rundown of 9mm terminology
9x19: Also known as 9mm Luger (the gun it was first chambered for), 9mm Parabellum (the original German name for it translates to "for war") and 9mm Nato. This is what you Glock is chambered for, when someone just says "9mm" this is usually what they mean. This is the most common of all the 9mm's.
9x18 Makarov: This is the round chambered in the Makarov pistol for which it was named. It is also chambered in certain FEG pistols, Polish P64's, CZ 82's, and a few other Eastern European pistols. This cartridge uses a case 1mm shorter than the 9x19 and a slightly larger diameter bullet. There was a 9x18 round that used the standard .355 bullet chambered in a few experimental Walthers, but I don't believe that round is in production by any major ammunition manufacturer.
9x17 Browning: This is the European designation for the .380ACP. Also sometimes called 9mm Short, 9mm Kurz (German for "short"), or 9mm Corto (Italian for "short"). This round is chambered in a myriad of small pistols such as the Walther PP series, Bersa Thunder, Mauser HsC, Beretta M1934, Colt M1908, FEG PA63, Kel Tec P3AT and so on.
9x21: This cartridge is not commonly seen in this country. It is ballistically the same as the standard 9x19 and has the same OAL but uses a case 2mm longer. This is a way for shooters who live in countries where the 9x19 is illegal (because it's a military caliber) to get around the rules.