You pull a shotgun trigger, you squeeze a rifle trigger.
The idea of squeezing the trigger on a rifle is to ensure the the force you apply doesn't move the rifle when the shot breaks resulting in a pulled shot that doesn't hit where you thought you were aiming. Like XTP said, when you shoot the rifle, you gradually apply more and more pressure until the shot breaks. A good way to break the habit of pulling or jerking the rifle trigger is to actually say out loud (on in your minds voice if you have people nearby
) "Squeeeeeeeeeeeeeze" as you place pressure on the trigger. If you do it right what you'll get is more "squeeeeeeBOOM!"
I know it sounds a little silly, but they actually teach this technique in Marine Corps Basic Marksmanship training (which is where I learned it). It's a way to make your mind concentrate on squeezing the trigger and not think about rushing the shot or anticipating recoil. Ideally, the pressure should be applied slowly enough that the shot surprises you a little bit.