The Glock 17C (C standing for "compensated") has a ported slide and barrel to signifcantly reduce muzzle climb during the firing of the pistol.
The Glock 17L is a competition version with a longer slide and longer barrel.
The Glock 17A is a variant of the Glock 17 specifically produced for the Australian market, to conform and apply with local laws regarding barrel lengths created after the tragic Monash University shooting.