Lasgun
The humble (and we mean humble) Lasgun is the standard-issue armament of the average Imperial Guardsman in the Warhammer 40,000 setting. Given the incalculable number of Guardsmen under arms and the sheer scale of the Imperium itself, it is safe to say that lasgun is probably the most common weapon in the entire galaxy that isn't some underhive piece of junk like stub guns, knives or other miscellaneous weapon.
The name is rather obviously a portmanteau of "laser" and "gun", but argument rages over how the word is meant to be pronounced. Many claim that the "las" should be pronounced "layz" or in other words say that it's pronounced with a long "a", as the first part of "laser", but others contend that this sounds retarded, and "lazz-gun" or a short "a" sound is a better pronunciation. According to Games Workshop, its name is pronounced "Laze-gun".
It is almost universally described in the fluff as discharging with a sharp crack (caused by the beam ionizing the air it travels through), but some authors describe the lasround as either a "bolt" as in Star Wars or as a "beam" as in Dawn of War. Other features of the weapon have greater variation - some Black Library works and items such as the Imperial Infantryman's Uplifting Primer describe the weapon as possessing a fully automatic firing setting, but this feature is not represented on the tabletop or in Dark Heresy - this can be explained away by virtue of the fact that there are many different patterns of Lasgun produced throughout the Imperium.
There is also some dispute as to the color of the "beam". Some fluff claims it to be blue, while games like Dawn of War portray it as red or something of a reddish-yellow. The popular Gaunt's Ghosts series of novels by Dan Abnett say that the Imperial weapons fire blue "beams", while the Chaos weapons fire red ones.
If we were to determine it scientifically, light comes off as different colours depending on it's frequency which is related to how much power it has. It stands to reason that Lasguns would fire different coloured beams depending on how much energy they have left in their power cell or what state of disrepair they're in. Under this model no colour is definitively right. Some wavelengths contain more energy than others, so assuming that the shot is relatively high energy it would likely be at a wavelength above the visible spectrum - meaning the actual damaging shot would be invisible. On the other hand it is possible that they might fire a tight, lower-energy beam nearly concurrently with the actual damaging beam as a sort of "tracer round". This could be adjusted to certain wavelengths to produce specific colors, which might serve friend-or-foe identification purposes to tell from which side the shots are coming from.
Why the Lasgun sucks
The lasgun is rather pathetic compared to the mainline arms and armour of most of the other armies of the setting, useful only by virtue of the fact that Guardsmen come in ridiculous numbers (except in comparison to Tyranids or Orks) and the application of statistical probability. The weapons are often derisively referred to as "flashlights", based on their individual usefulness, although this is more of an example of how ridiculously tough everything else is in the setting.
The 6th Edition rulebook describes the lasgun as severing limbs at close range but further away it only goes about as deep as the liver (it'll roast that liver good before it's done, though), explosively flash-boiling said liver and all other squishy bits in the laser's path.
Why the Lasgun is still being used
Although better known as a "flashlight", some players also call it "The AK-47 of the 41st Millennium", albeit ironically. Instead of being reliable, tough, relatively inaccurate and having a tendency to show up in the hands of just about everyone, it's reliable, common as fuck (like 100 for every human in the galaxy common), pretty damned tough, and accurate but that is only by 40k standards. ...So, yeah, I guess it is the AK of the 41st Millennium.
The lasgun has very low recoil, especially in comparison to a bolter or even an autogun. The powerpack that the weapon uses can fire about a hundred shots before running empty and can be easily recharged through any standard Imperial power supply, direct sunlight, and even heat. A relatively common practice of Imperial Guard units cut off from supplies is to place their Lasgun power packs in open fires, although this does lower the pack's lifespan considerably and is frowned upon.
The weapons are rather more useful in Dark Heresy than they are in the tabletop wargame, as their reliability, availability and plentiful ammunition become real considerations and they are rather more useful against the human foes that an agent of the Inquisition is likely to face.
One last thing about Lasguns is that they can be used as last-ditch grenades by basically making the entire power pack empty into the gun without ever actually firing it. It turns the Lasgun into an explosive comparable in power to a Melta charge. It can really fuck up the shit of even a Chaos Dreadnought who thought they were going to make some Guardsmen go squish and instead find that their face gets blown off.
Uses for the Lasgun
-Warming Soup
-Cigarette lighter
-Changing TV channels
-Selling to buy a new weapon
-Pissing off thunderhawk pilots
-Shining in the enemies eyes
-Throwing at people (may cause more damage than firing)
-Burning ants
-Paperweight
-Using as a laser sight for a boltgun
Other laser weapons
The Lasgun also comes in carbine, pistol, and sniper ("long-las") varieties, and the Imperium fields many other weapons based on the same technology, such as the Hellgun (a powerful Lasgun powered by a backpack power pack), and the mighty Lascannon, and the even bigger Volcano Cannon, and then the even bigger Defense Laser, and the bigger still Las weapons mounted on starships.
Sniper Rifle
The term "sniper rifle" is fairly loosely used in the Imperium; many sniper rifles are simply over-powered lasguns called long-las rifles, while many others (such as those used by Space Marine Scouts) are high-velocity slug throwers which have more in common with autoguns than they do with energy weapons. Then there are other more exotic variants firing poisoned darts or even stranger payloads. All of these variants have pretty much the same effect on the target, unless you play older editions or use special characters who have rules that say otherwise. Or play Dark Heresy.