From http://en.wikipedia.org/wiki/Glock
Safety mechanisms
Glock pistols use an internal safety mechanism with three components, with no external thumb activated safety switch as might be found on traditional-design pistols. Glock calls this the “Safe Action” system. All three safeties are disabled one after the other when the trigger is depressed. They are:
Trigger Safety: An external lever mechanism contained within the trigger that prevents the trigger from moving unless the lever is depressed.
Striker Safety: A spring-loaded pin attached by an extension bar to the trigger assembly blocks the striker from striking the primer of the cartridge until the trigger is pulled.
Drop Safety: The far end of the same extension bar locks the striker into place from the rear until the trigger is pulled.
Similar systems for internal safeties have since become standard for many major manufacturers of semi-automatic pistols. However, Glock pistols, like any other firearm, can discharge and cause injury or death if the operator accidentally or negligently manipulates the trigger. The absence of a traditional safety switch means that Glock users who intend to carry the gun on their person with the chamber loaded must be cautious (as they should be for any type of firearm) of keeping their finger off of the trigger when holstering or unholstering the gun; however, the firearm will not discharge if dropped, requiring a deliberate trigger pull to discharge the firearm.
In 2003, Glock announced the Internal Locking System (ILS). The ILS is a manually activated lock that is located in the back of the pistol’s grip. It is cylindrical in design and, according to Glock, each key is unique. Group key hierarchic solutions are available for law enforcement agencies. When activated, the lock causes a tab to protrude from the rear of the grip. This is done to give both a visual and tactile indication as to whether the lock is engaged or not. When activated, the ILS renders the Glock unfireable as well as making it impossible to disassemble. When disengaged, the ILS adds no further safety mechanisms to the Glock pistol.
The ILS is available as an option on all Glock pistols except for the G36, but not all ILS-equipped Glock pistols are carried by distributors nor imported with the option. The most commonly available Glock pistols with the ILS are the G17, G19, G22, G23, G26, and the G27.
The trigger safety.
The internal safeties.
Optional extra safety the “standard” thumb safety.
A 100 round mag for the Glock, very useful.