Arty rounds are ordnance fired at your position by Wespes, Arty Strikes and Precision artillery strikes. They have a wide gib radius, and lot of fragmentation. Utterly lethal, but do contribute towards the Shellshock Ribbon which removes 1 round from each strike. An artillery round that does not explode upon impact will leave a dud round that can explode at any moment.
There are many types of artillery rounds including ground impact rounds, HE (high explosive), incendiary, frag rounds, gas rounds (WW1, currently banned from warfare), anti-tank artillery, smoke rounds and nuclear (not used).