also, National Match isn't specifically a trigger; normally refers to a complete rifle that is legal for use in the Civilian Marksmanship Program's matches, or the Service Rifle class in NRA matches
this mostly means that externally, they resemble bart's description of an M16A2 (minus the fun 3-round burst part, of course). if a rifle is marketed as a NM, that also typically means it has match type parts in it. that normally means a better than average barrel, sites with a much smaller peep on the rear and much more granular windage/elevation adjustments, and often a 2-stage trigger