"Decepticons, attack" is the general order for an attack among the Decepticons. It usually signals the opening of hostilities or an order to begin a concentrated assault against their intended target, be they Autobots or organics. The order is usually given by the Decepticon leader, a designated mission commander, or whoever else is in charge.
Generation One cartoon
As leader of the Decepticons, Megatron was the one who most commonly gave the order of "Decepticons, attack!", becoming one of his signature cries. Usually, however, this would be followed by an order of "Decepticons, retreat!" at some point within the next five to ten minutes.
After Megatron was reformatted into Galvatron and driven insane, he still gave his usual order of "Decepticons, attack!", but this time, it had a new meaning. It was not so much an order to attack as a warning that the Decepticons were going to be attacked by their psychotic leader.
The Predacon, the claimed Decepticon descendants, also use a phrase similar to this terminology in battles, mainly by Beast Wars Megatron. However, they would often switch around words to get their order through, from very sophisticated speeches to the blunt "back to base".
Once Megatron finished dismembering Jazz, he gave the order of "Decepticons, attack!". There is little doubt that this was music to the audio sensors of Blackout and Devastator (Blackout because it meant that Megatron was in charge again, Devastator because he got to kill things).
During the noble Decepticons' plan to destroy the evil Autobots' Ark, the heroic Megatron gave the order of "Decepticons, attack!" once they were assembled. Just goes to show that the more things change, the more they stay the same.