So just to sum-up what guys above said:
unit - order x unit to atack-ground y - orders unit to attack target point but with splash AoE, what works only if unit got splash damage, if not it will order unit just to attack - move
unit - order x unit to atack-move to y - gives an 'attack' order' to unit. Attack order tells that unit will move forward to tarter point. The difference is that enemies approached disrupt move order and let your unit attack enemies instead. This is repeated with each enemy approached, untill it gets to destination point.