Captain Jack (Updated: 9/27/2016):
Use of a selective fire system allowed the Model 18 to provide voluminous fire capabilities from within a compact package. The Model 18 was chambered for the universally accepted 9x19mm Parabellum pistol cartridge which had also been adopted into submachine gun lines. The 9mm cartridges could be loaded into the Glock 18 by way of the standard Glock 17 magazine or the special extended 33-round capacity magazine most common to the Glock 18. The Model 18 could fire its 33-round magazine in seconds while featuring a 1,200 round-per-minute rate-of-fire. Barrel length was approximately 4.6 inches. Sighting was through the standard Glock front-and-rear arrangement. Spent cartridges were ejected through a typical top mounted ejection port, the slide recoiling rearwards with each successive shot.
Of course this sort of compact firepower came at a price. Since the Glock 18 was essentially a high volume pistol (modified into the machine pistol role), recoil was extraordinary particularly to the untrained shooter. As such, the model was typical fitted with its own adjustable skeletal stock to permit firing from the shoulder (the second hand gripping the extended magazine) with more control and, therefore, more inherent accuracy. However, its use of the 9mm Parabellum pistol cartridge and handgun frame kept it more or less a short-to-extreme-middle ranges weapon system.
A version of the Glock 18 with add-on recoil compensator was introduced in the "Glock 18C" model and, like other Glock guns, was engraved to showcase the change along the forward portion of the slide.