Glock 18 is a select fire version of the 17 - the 17 is a full sized semi-automatic pistol, chambered for 9x19mm.
32 is compact pistol chambered for... something that isn't 9mm... oh wait, .357 SIG.
Googling "Glock 18" brought me this:
http://en.wikipedia.org/wiki/Glock_pistol. Scroll down to variants and read.
If in doubt, Google it or something like that. Are you talking about airsoft Glock GBB's or what? Because you do know that there are various airsoft replicas that are produced from different manufacturers, some of which are not cross-compatible with each other, right?