Unlike the Lasgun, the Lascannon is a flashlight that may be considered badass. Lascannons go FREEEM! or FOOSH! There is an ongoing debate on the pronunciation of "lascannon." As lasers are LAY-ZERZ, some players know the lascannon as the lays-cannon. YOU WISH IT COULD SHOOT POTATO CHIPS! It can't be called a debate when the powers-that-be already stated that it is pronounced like laser but with the "er" removed, but nobody at GW seems to actually stand by this ruling as several official videos, interviews, and audiobooks pronounce it the other way.
In all seriousness, a laser causes what it touches to explosively flash into plasma if the laser is strong enough. A normal lasgun is strong enough to blow an unarmored human nearly in half and, due to the way lasers work, can damage pretty much anything eventually. So, a lascannon does the same with far greater effectiveness. The lascannon beam doesn’t necessarily need to penetrate its target either. In many cases, the explosive damage caused by the hit itself will be enough to critically damage a target. The lascannon is, like the lasgun, a laser and does not "cut" or "bore" into a tank so much as make part of the vehicle explode with the force of a bomb. This thing will fuck your shit up and fuck it hard. Think miniature pyroclastic explosion. At your feet or your cover.
The lascannon is often placed as a phallic symbol on many Imperial Guard Leman Russes, because they thought the battle cannon was not enough. Spess Mahreens often use them in their Devastator Squads, because they are simply better than their Guardsmen counterparts, and want to make sure that's clear. But like their Guard counterparts, they like to stick them on all the tanks they have, because they are Spess Mahreens, and they're cool like that. Being an energy weapon, the Lascannon shares many drawbacks of similar weapons fielded by the Imperials in that it's not terribly energy-efficient, but it's hard to find a better anti-tank weapon or tool for reliably hurting lone high-threat units on the battlefield. Seeing as it is Strength 9, AP2, and Heavy 1, it really has no other use, save for what is ultimately sniping, only that vehicles and monstrous creatures are also on the list of targets, while squadded commanders are not.
Lascannons use single-charge power packs similar to las-gun power packs, but the size of a car battery and good for one shot. This means they have to be reloaded after each shot, like a missile launcher. Tanks with lascannons use a bank of these packs to help distribute the charge and recharge spent packs between volleys. They are most visible in the Predator tank and Sentinel walker. Theoretically, a Space Marine could hook up a scaled-up hellgun to his miniature nuclear reactor backpack and just let loose with some sort of hellcannon, but that would mean thinking and thinking is heresy.
The Lascannon is primarily designed for anti-vehicle combat, but it also sees use against monstrous creatures and the errant battlefield target that a commander needs to remove right the fuck now, such as isolated enemy characters. While it's not by any means the most powerful anti-armour weapon on the tabletop, the lascannon is still quite effective, and in the case of Imperial armies, it is available in quantity. Just bear in mind that when forced to operate outside its preferred role of burning down tough, high-priority targets, the lascannon is far less efficient.
On the table, the lascannon generally represents the minimum level of firepower necessary to be at least a nuisance to anything. Given enough time you could lascannon a baneblade to death; and if you brought ENUF of them you might be able to do it before losing your whole force. As a serious armor killing option, lascannons rely on bringing them in numbers. Both Annihilators, the Predator and the Leman Russ, bring at least a pair, and their hulked out cousin brings a ludicrous eight.
It's hard to go wrong with the Lascannon. Ask for it by name.
The Lascannon's beefier brother.
The Heavy Lascannon is one of the three primary weapons that is used by the Imperial Guard Field Ordnance Battery. Considered as the premier tankbuster of the family, the Heavy Lascannon is a Lascannon that was given beefier barrels, stronger lenses and a more powerful battery to make sure that the target stays extra dead. Due to its larger size and power upkeep, the Heavy Lascannon so far can only be fielded as a field artillery. Regular Lascannons are at least relatively compact enough to be mounted as secondary weapons for most vehicles, not so for the Heavy Lascannon however, as it is just too goddamned heavy and bulky to act as a proper sponson.
Crunchwise, the Heavy Lascannon is a Heavy 2 weapon that is S10, AP-4, D3+3. A much nastier brother of the regular Lascannon. Its ridiculously high strength and great AP mean it will be penetrating most armour. A field of three of these monsters can make TEQs cry in their sleep. Of course, that Heavy Lascannon users need to be in the line of sight, making them quite vulnerable to counter-fire. But holy shit do tanks suffer critical existence failure once these bad boys show up.
Solex Heavy Lascannon
A variant of the already powerful Heavy Lascannon.
According to the Sollex Sub-Cult of the Auxilia Myrmidon, this weapon was created by the Pontifex Technis Ayahpana during the Age of Strife. These Lascannons operate on an order of magnitude greater than standard Imperial laser weaponry. Their firepower is unmatched for their size, although they require more resources and expertise in their manufacture. Each weapon is treated with reverence by the Tech-Priests who maintain them.
On the Horus Heresy tabletop, the Solex is a 60" S10, AP2, Heavy 1 anti-tank weapon. It would have been considered a fantastic weapon if it wasn't a one-shot weapon. Compare it to the Krios Venator which costs 100 points less and gets 4 x S9 AP2 shots with Ordnance as well. However, with enhanced targeting, you will only be missing on a 1 and you will be lowering cover saves by 1. So against any mech army, this gun has a decent chance to render most tanks impotent for a turn by making them snap-shoot or even blow them up.