A Lasgun is in effect a laser rifle. Laser weaponry is fielded mostly by the Imperial Guard. A lasgun is weak on it's own but increases in power when used en masse.
The lasgun is very easy to produce and maintain; these qualities have made it the workhorse of the Imperial Guard. Lasguns are powered by an ionised pack, which is quickly and easily rechargable from stations, heat or, in desperate circumstances, by placing it in a fire (although this tends to shorten the lifespan of the pack).
Lasguns are very easy to maintain and are incredibly reliable, which is why many Imperial Guard veterans prefer the lasgun over more powerful and esoteric weapons which could fail them when needed the most.
Sergeants and Commissars use a Laspistol.