A laser cannon was a large laser device used for industrial mining or as a directed energy cannon weapon aboard spacecraft and other installations or as ground artillery. (TOS: "The Cage"; ENT: "Demons"; TNG: "Conundrum")
Starfleet's surface-deployable laser cannons that utilized energy transferred from a starship in orbit illustrate the late maturity of the technology. Showing a clear design lineage, late-model Starfleet cannons were similar in structure and scale to the United Earth industrial lasers used in mining operations as far back as 2155. (TOS: "The Cage"; ENT: "Demons")
Around the mid-23rd century, Federation abandoned the military use of laser technology in favor of phaser technology. In 2368, Lysian Central Command was considered by Starfleet to be technologically a hundred years behind the Federation, as it was armed with four laser cannons. (TNG: "Conundrum")
According to the Spaceflight Chronology, the invention of advanced ship-mounted laser weaponry was the key technology credited for the victory in the Earth-Romulan War. Laser weaponry was eventually abandoned by Starfleet because, while it did penetrate deflector shielding easily, lasers had very little impact force to do actual damage to a hull, compared to particle weapons.
It should be noted that the directed energy device in "The Cage" (and "The Menagerie, Part II") was never mentioned on screen to be a laser cannon. In fact, the first draft outline of "The Cage" describes the device as one of the vessel's huge matter converters, focusing its conversion on the rock. The term laser cannon, however, began to be used soon after by Gene Roddenberry and other members of the production staff, according to behind-the-scenes information from The Making of Star Trek and Star Trek Encyclopedia.
The laser cannon shown in "The Cage" (and "The Menagerie, Part II") was built atop a camera dolly, with its forward arm supporting the firing mechanism rather than a film camera. The weapon's rear control panel hid a box used to hold counterweights, which ordinarily balanced the weight of the camera mounted on the arm. Animation, produced by the Howard Anderson Company, was used to show the laser cannon firing. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD)
The directed energy mining drill in "Demons" was also never identified as a laser cannon on screen or in any behind-the-scenes statement. The prop is, however, a "laser cannon prop"  that was rented from Modern Props for the episode. It was previously seen as a 24th century Minosian advanced cannon weapon that had been used to melt tritanium in TNG: "The Arsenal of Freedom", speculated by Riker to be a demonstration model for a potential buyer.