Unfortunately, after reading through the replies you will still be confused because there are so many dang wrong answers. Easiest way to explain it is impact is base damage a weapon does, it is by definition the impact each bullet has on its target. Impact is not comparable to other weapon types. To whoever disagrees compare snipers impact to auto rifle and try to tell me one auto rifle bullet does similar damage as one sniper bullet. Attack is a modifier or boost to damage, it is not considered in crucible. Like many people have said 331 attack is best for level 33 bad guys. 200 -331 will be same thing against level 20 bad guys. Basically a 300 attack weapon will not do 100% of it's potential damage when shooting a level 31 or higher, the higher the bad guy the worse the dropoff to damage. Also for the record is somebody mentioned fire rate ignore their comment. Fire rate affects time to kill or damage per second, neither of which you asked about so it's just adding confusion. No offense to people who mentioned it but obviously by reading these replies you can tell impact/attack is still the biggest source of confusion in the game why make an explanation even more confusing.