The laser (Light Amplification by Stimulated Emission of Radiation) is the most basic of modern battlefield energy weapons. Essentially when a laser is fired, it emits a burst of concentrated heat, which can melt through even the hardest of military grade armour with relative ease in a fraction of a second. The only thing stopping the laser from being completly unstoppable is the danger to the optics used to aim and focus the shots when in use for more then a few seconds at a time.
No requirement for ammunition means that lasers are often mounted on recon Battlemechs, so that they may operate without resupply, often behind enemy lines. This advantage carries over to many light mechs who simply do not have the space to carry heavy and bulky ammunition.
The heat generated both by the weapon and the engine in response to the weapons need for energy can be hazardous to a 'mech without heatsinks that are capable, or damaged 'mechs who have lost heatsinks or engine covering.
Lasers have also had improvements made to range in the form of Extended range lasers which fire further at the cost of higher heat generation and damage, Pulse lasers which use rapid cycling high energy pulses to generate several laser beams, similar to machine gun fire, and Heavy lasers which pack a bigger punch at a similar range, also at the cost of higher heat.