The Glock 18 is the only factory installed
fully automatic capable pistol to come out of the
Glock company as of this date. It is basically a
Glock 17 with a rotary fire selector switch in the left side of the slide that allows for selection between
semi and
full auto fire.
A quote from remtek.com
As we've seen the development, design and worldwide
acceptance of every Glock model and caliber, one model
has all but been ignored - the G18/G18C. These are Glocks
that offer the option of fully-automatic fire. The Model
18 has three external changes from the original Model
17: a left side, slide-mounted fire control or selector
switch; a barrel that extends past the front of the slide and three horizontal and diagonal cuts that run across
the top of the barrel to act as a compensator (the same
design used on the 17L and 24 compensated models).
end quote.
The mentioned difference in slides to prevent interchanging of parts between the 17 and 18 are most likely put there as protection from legal liability. It is quite simple to create a selector switch for your Glock if you can understand the diagrams on USA Patent Number 5,705,763.