"Laser" stands for "Light Amplification by Stimulated Emission of Radiation", or a device that focuses an amplified beam of light on a small surface area. Lasers cause damage by firing an intense beam of light at a target, flooding concentrated energy in the form of heat, which can melt material and overwhelm heat-sensitive electronics. Like other beam weapons, the large laser has a duration of fire which requires the pilot to hold the beam on their target for a short period.
The Large Laser increases the power of the beam when compared to the Medium Laser, allowing it to reach to further ranges and cause more damage. The increased heat and weight requirement generally means only heavier 'Mechs will be able to use the Large Laser effectively.
When compared to the Large Pulse Laser, the standard large laser deals less damage, but has a much greater range, weighs less, and causes less heat build up. The ER Large Laser is identical to the large laser in almost every respect except that it fires farther in exchange for even greater heat build up.