The Cutter Beam is an ability used by Megatron. Usually voice-activated, it is fired from the crest atop Megatron's head. It is normally depicted as a yellow or green beam of energy, but it sometimes appears as lightning-style bolts or even short bursts of energy. It may or may not be related to a particle beam. Who knows?
When Megatron's kidnapping of Dr. Onishi was interrupted by Optimus Prime, Megatron used his Cutter Beam to destroy a building, burying Prime. Later, Megatron used his Cutter Beam on the Autobots after the Predacons failed in a mission to destroy a train. The Cutter Beam was used several times after that, the last occasion being when Megatron used it to blast Prime and Ultra Magnus. Why Megatron never used it again is unknown.
Interestingly, rarely was the Cutter Beam depicted with stock footage. Instead, it changed appearance nearly every time it was used.