The Lasgun is an energy based weapon, and is the most common and widely used type of laser weapon in the Imperium. It is standard issue for all Imperial Guard infantry and most lower-ranked officers.
Lasguns are reliable, easy to maintain and produce, and are readily available on most Imperial worlds. They have longer range and higher ammunition capacity than a Laspistol (the laspistol power packs having roughly 80 shots before depletion, compared to roughly 150 shots for a lasgun), but are nowhere near as powerful as a Lascannon. They also lack the rapid-fire quality of the Multi-laser or the Hellgun.
The lasgun uses the same basic technology and operates along the same lines as other laser weapons, emitting a beam of focused light. The high amount of energy in the beam causes the immediate surface area of a target to be vaporized in a small explosion. It is a relatively unimpressive weapon when compared to other weapons in the Imperium, but is still capable of cleanly severing limbs or piercing the power armour of a Space Marine (but only through a vulnerable spot in the armour). Most designs have iron sights mounted along the top of the weapon. It is powered by a small, rechargeable power pack located beneath the weapon, in front of the trigger guard.

Source: https://wh40k.lexicanum.com/wiki/Lasgun