9x19mm NATO  

one of the alternative names for 9x19mm Parabellum used by numerous pistols, sub machine guns and pistol-calibre carbines. the origional was developed by the germans for the 9mm luger pistol, hence the americans also knowing the round as 9x19mm luger. it's all the same thing.
The NATO part comes from the fact that it is the standard pistol bullet used by the armies of NATO countries, such as the USA, Britain, Canada and France (to name but a few)
notable guns which use this round include the classic WW1/2 German Luger, The H&K MP5 series, The UZI, The Browning Hi-Power (P38) and some glock models.

person 1: hey man, is that glock using .45 ACP?
person 2: nope, it uses 9x19mm NATO

9x19mm NATO by phaserrifle March 24, 2009

9x19mm NATO 

Could be mistaken for the 9x19mm Parabellum; it's the caliber for H&K's infamous MP5 series, especially the MP5N (MP5 Navy).

"Someone correct me if I'm wrong here; I only looked up this caliber on the internet! >_<"
-me

9x19mm NATO by Dave February 3, 2004