Set trigger
A set trigger allows a shooter to have a greatly reduced trigger pull (the resistance of the trigger) while maintaining a degree of safety in the field. There are two types: Single Set and Double Set.
Single set trigger
A Single Set Trigger is usually one trigger that may be fired with a conventional amount of trigger pull weight or may be 'set' by usually pushing forward on the trigger. This takes up the creep in the trigger and allows the shooter to enjoy a much lighter trigger pull.
Double set trigger
As above, a double set trigger accomplishes the same thing, but uses two triggers: one sets the trigger and the other fires the weapon. Set triggers are most likely to be seen on customized weapons and competition rifles where a light trigger pull is beneficial to accuracy.
Double set triggers can be further classified by phase.[4] A double set, single phase trigger can only be operated by first pulling the set trigger, and then pulling the firing trigger. A double set, double phase trigger can be operated as a standard trigger if the set trigger is not pulled, or as a set trigger by first pulling the set trigger. Double set, double phase triggers offer the versatility of both a standard trigger and a set trigger.
http://en.wikipedia.org/wiki/Trigger_(firearms)