A laser was, in a description posited by Leela and approved by the Fourth Doctor, a very powerful light. (TV: Horror of Fang Rock) Many lasers were harmless and used in some games, (TV: Warriors of Kudlak) but others were used as deadly weapons. During their campaign to recruit warriors, the Uvodni utilised harmless laser weapons in their interactive laser-tag game Combat 3000. However, on board their ship, the Kudlak warriors carried deadly blaster rifles that fired a yellow laser burst.
Some weapons, such as Sontaran lasers, were called "lasers" despite travelling slower than light. (TV: The Sontaran Stratagem)
Lasers could be used for a defensive purpose, as when the Tenth Doctor, Donna and Jenny came across a grid of heat rays in one of the tunnels underneath Messaline. (TV: The Doctor's Daughter) Lasers were also employed for medical purposes, in surgery, such as in laser scalpels (TV: Mindwarp) and laser saws. (TV: Day One)
Examples of laser weapon use
The Cybermen were armed with a wrist-mounted laser blaster that fired a red blast. (TV: The Pandorica Opens)
The Daleks used gunsticks which fired laser rays which could be modified into hand-held Tommy rifles. (TV: Evolution of the Daleks)
The parallel Torchwood developed laser weapons, which destroyed Cybermen and could blast through a Dalek's shield. (TV: Doomsday) These weapons were later upgraded, to the point that they could destroy Daleks in one shot. (TV: The Stolen Earth)
Jack Harkness had a compact laser deluxe. (TV: Bad Wolf) Jack once used components from a pair of androids to build a high-powered energy weapon out of a defabricator. (TV: Bad Wolf)
The Sontarans utilised laser rifles during their invasion attempt. In addition, Sontaran batons fired a laser that could cripple their victims. (TV: The Poison Sky)
The Judoon carried hand-held blaster pistols which fired a red disintegrator beam. (TV: Smith and Jones)
The Toclafane had laser blasters built in to their casings. (TV: The Sound of Drums)
Chantho used a laser pistol to shoot the Master. (TV: Utopia)
Eva Saint Julienne used a laser pistol to shoot the Jagrafess. (TV: The Long Game)
Novice Hame carried a laser gun for protection when searching for the Tenth Doctor. (TV: Gridlock)
The Saxon Master occasionally used a hand-held laser gun. (TV The Claws of Axos, Colony in Space) Later, he had a laser screwdriver, capable of killing. (TV The Sound of Drums, Last of the Time Lords)
The Slitheen-Blathereen family used bracelets which fired orange lasers. (TV: The Gift)
The Merons carried a trident-shaped weapon which shot 3 green beams. (TV: Liberation)
The Korven had a laser weapon on their palms. (TV: The Korven)
Ahab made use of a laser lasso. (TV: The Bounty Hunter)
CCPCs were armed with a palm-mounted laser blaster that fired a red beam. (TV: Aeolian)
The Chameleons threatened the Second Doctor's and Samantha Briggs's lives with a laser beam. (TV: The Faceless Ones)
Behind the scenes
In the video game Attack of the Graske, which this wiki does not consider to be a valid DWU source as it has multiple endings which we cannot qualify through any mechanism which one is the "correct" or "canonical" one, Graske on the planet Griffoth were shown to be armed with tiny laser pistols which fired purple blasts to combat intruders.