An abbreviation for Light Amplification by Stimulated Emission of Radiation, the laser was discovered on Earth in the early 20th century and came into widespread use as a weapon during the mid 21st. Most lasers use a form of crystal or gas in which the electrons have been raised to an excited state. Mirrors are then used to reflect stray photons back and forth through this medium, provoking a cascade reaction which releases a burst of light. The beam emitted from a laser is monochromatic, coherent, and has a very high intensity. High energy lasers are thus able to focus large amounts of energy onto very small areas, which can result in considerable physical damage.
Lasers remained in widespread use for some two hundred years, and were the main armament of the Daedalus and Constitution class starships during the early years of the Federation. By 2255 the standard issue hand laser was capable of blasting through considerable thickness' of rock. A larger artillery style ground cannon could provide heavy fire - this weapon could be rigged to run from remote power sources if required, supplying sufficient power to blast half a continent.1
Between 2255 and 2265 lasers where replaced in Starfleet service with the much superior phasers.2 Today lasers are not a significant ship-to-ship weapon, as the navigational shields of a Federation starship are immune to this form of energy.3