Basic Trigger Terms
Blade
The (Blade) is the area on which the shooter applies pressure to fire the gun.
Break
The (Break) is the point at which the mechanism releases the hammer to hit the firing pin.
Double Action
Typically  applied to pistols, a double action trigger will move the hammer backwards, delivering energy  to strike the firing pin. DA triggers  will also release the hammer once enough energy is stored.

 GRITTY  
 A (Gritty) trigger feels like metal sliding on sandpaper  and requires more pressure to release the mechanism. 
Over-Travel
Any rearward movement of the trigger after the (Break).
Reach
The (Reach) is the distance from the (Blade)where your finger  would apply pressure, to the front of the grip.
Reset

  To (Reset) a trigger usually means allowing it to return to its most forward. 
Shoe
A (Trigger Shoe) is a piece of material that is attached to the blade of the trigger in order to change some characteristic of the trigger.  
Single Action
(Single Action) triggers, do one thing, they release a round; once pass the (Break).
Slack
 (Slack) is the distance that the trigger travels backwards before resistance is felt. 
Smooth
Unlike a (Gritty ) trigger, a (Smooth) trigger has steady resistance and the mechanism slides backwards without any extra effort. 
Stop 
 (Stops) are a physical  piece of metal machined into the trigger or an adjustable screw. that limits the over-travel of the trigger. 
Two Stage
(Two Stage) triggers, have a small amount  of (Slack) built into the trigger. The (Slack) gets taken up, and stops right before the  (Break (Stage 1)). It will stay in that state until a little more pressure forces the trigger pass the (Break (Stage 2)).

 I know I am leaving out more definitions, but this will give you a good understanding of the basics.