The Glock 18 is a pistol produced and manufactured by Glock of Austria. The G18 is very similar to the Glock 17 however on the slide has a fire selector switch that enables the pistol to fire in fully automatic or semi-automatic modes.
Rotating the switch to the top switches the pistol to semi automatic mode, however rotating the switch to the bottom allows for fully automatic mode.
The Glock 18 has an identical appearance to the Glock 17, however the g18 has the addition of the fire selector. Due to the fully automatic mode, the internal infrastructure of the Glock 18 is markedly different from the Glock 17, and the parts are not interchangeable. This was purposefully performed by Glock in order to ensure that the Glock 17 was not a semi automatic version of the Glock 18, however the two would be considered separate and distinct pistols.
Some jurisdictions and countries have laws that place unique and special restrictions on semi-automatic variants of fully automatic weapons, and if the Glock 18 were to be essentially identical to a Glock 17, the Glock 17 would fall within the purview of those laws and be unavailable in those countries and jurisdictions.
The Glock 18 is in the United States an exclusively law enforcement use pistol that utilizes standard Glock 17 magazines, holding either 17 or 19 rounds. The G18 can be used with special 31 or 33-round extended magazines, however this configuration is less common. There have been known to be a few Glock 18s exclusively and privately owned in accordance with the National Firearms Act.