You see this man?
He is just like you.
A factory worker, a farmer, a trader or a storekeeper.
A father, a brother and a son.
This is an Imperial Guardsman, one of the billions of soldiers that fight and win in the name of Holy Terra, the Imperium and the God-Emperor.
This document will inform you of them. Ave Imperator.
The Imperial Guard, known by its official name in High Gothic (or GW's works, take your pick) as the Astra Militarum, is the basic ground fighting force of the Imperium of Man. Due to the nigh endless supply of recruits, the dozens of warzones it occupies, secures and creates, and what with casualties, recruitments, promotions, so on and so forth reaching into the billions every single day, it's quite literally nearly uncountable. This fact is exaggerated by the vast distances of space, the unreliability of Warp-based travel and communication, and also by the simple human bureaucracy of the Imperium.
In the tabletop wargame of Warhammer 40,000 (currently in its 7th edition) they are somewhat to the wayside, as a high number of models coupled with generally low stats for them in return for low points cost and a wide, wide number of options. Most put them aside in favor of the Space Marine armies, or Orks and Tyranids if you still prefer massive numbers in your armies.
The counterblow of all this delivered by Emperor-blessed pimp hand, though, is delivered by a small update in the 6th edition codex. You can get the Imperium's main battle tank, the rapetastic Leman Russ, for only 500 points and as a troop choice. Given how armour units usually have a special category of its own and costs around 800-900 points for each tank, this means that the Russ and all its delicious variants can be fielded en masse and in a storm of battle cannon fire.
The equipment of the average Imperial Guardsman varies wildly and impossibly. Since the Guardsman's loadout is determined by commanders of the regiment, and due to the fact there are tens of thousands of such Guard regiments, the commander of each varying wildly in tactical acumen, this results in there being no real standardized loadout, except for the very basics.
The lasgun, body armour, and personal comms beads are the three things which seem to be default across most high-end regiments, while Penal Legions such as the Savlar Chem Dogs will probably just nick anything they can off dead enemies and not-yet-dead allies alike.
The lasgun, standard-issue to all Guardsmen, is perhaps the singular most reliable and widely used weapon in the entirety of the Imperium. Mass-manufactured at every single Forge World and supplied to all non-specialized forces, it sets the benchmark for directed energy weapons. Each forgeworld offers different designs for different purposes - the Death Korps of Krieg, for example, favoring accuracy over long range due to their doctrine of meat-grinding trench warfare are equipped with the works of their nearby Forge World Lucius, while the Elysian Drop Troops, supplied by Forge World Acctra, favor a compact, high-rate of fire design which has the firing chamber set back into the stock in a bullpup pattern. The Galaxy-pattern is the most common all across the Imperium. Ryza's never done things the right way, but it seems to be working for them.
A fun upside is that the technology and measurements and internal design of all laser weapons is the same. Yes, you read that right. Battleship-grade weapons are of the same design as the standard infantryman's, albeit upscaled.
Other varieties of lasgun include the more compact laspistol, a lengthened and higher-powered version called a long-las, hotshot lasguns which sacrifice powerpack capacity in exchange for firing power (sometime even going all the way up to Las-lock weapons, made for worlds contacted before automatic weapons had been discovered), the high-capacity, high-power, high-end hellgun which combines accuracy, firepower and rate of fire in exchange for being fed by a backpack generator, the mighty anti-vehicular lascannon, the gigantic Volcano cannon and Turbolasers mounted on war Titans, all the way up to battleship-scale weaponry.
Due the the Guard's numbers and nearly inexhaustible resources, it goes without saying that they're more for massive deployments and campaigns rather than individial, high-stakes battles. Though there are many forces within the Guard for said type of warfare such as the Militarum Tempestus and various high-profile regiments (or generally any Cadian regiment), the Imperium tends to conserve those and favor the Adeptus Astartes for matters of supreme importance.
Large-scale warfare favors the Guard in the fact that their main strengths are at their topmost when in the hard grind of attirition warfare. Trench warfare - or at least, WWII-era trench warfare - is for the most part frowned upon, as most competent regiments employ fluid command dynamics, individual initiative and squad-based operation.
For the average Guardsman, life is harsh.
Two months of training, one month in transit. Expected service life? Fifteen hours.
In the field of battle, whatever the odds, the chance of death is very high indeed. The Guard has no concept of pensions or retirement, birthing the quote "Only in death does duty end."
This means that for one's entire life they will be registered within the Imperial Guard. Over the course of said career, one will eventually have encountered the entirety of what the galaxy has to offer them if they would life long enough. This results in some extremely hard individuals, colloquially referred to as Veterans. Much like modern militaries, they form the real tile grout of the Guard, aside from the fresh recruits which have nearly no knowledge of how things work, whom are referred to as Frakking New Guys.
Regimental organizations vary in local titles and internal rankings at the direction of the officers, but for the most part it's simple enough. There is a singular senior officer per regiment, Lord Generals for multiple regiments, Commissars within the regiment dedicated to maintaining discipline and morale, and junior officers purely for command purposes.
The Commissar is the political and disciplinary officer of the regiment, and thus is charged with maintaining the morale of the men (or women) within it. This extends to all ranks and files, made possible by way of the Commissar being technically separate from the ranking system.
Their duties are unrewarding, and their life is particularly dangerous even by the Guard's standards. They're made to wear their iconic headgear which make them easy to spot by snipers, should they acquire the dislike of the regiment they will be the first targeted in the event of a mutiny. Various elite or death-world forces (the Catachan Jungle Fighters being the definitive of this) commit 'accidental' murder of disliked or incompetent Commissars, the practice being known as fragging.
It is to be noted that summary execution is meant to be an absolute last resort, much like Exterminatus. Far too often media and popular opinion paints the scions of the Commissariat as arrogant pricks who execute just for the hell of it. These are the sorts of officers who do not live very long, as doing so weakens the entire regiment and pisses off everyone. And being in battle amidst a regiment of Guardsmen that hate you is its own punishment.
More Commissarial artwork. Yes, it is the Commissar being shot through the hat for cowardice and lack of discipline in the directly above.
All in all, the life of the Imperial Guardsman is a harsh one. You are mass-recruited by the million and removed from your home world which you will likely never see again, ground through two months of awful training, sent on a one-month or ten-year voyage across space via ripping through a literal hell of a dimension in which quite literal daemons want to tear apart your soul and flesh alike if the technology-worshipping weirdoes in red robes don't maintain a part of the ship which runs on literal sorcery and if the psychic imprisoned inside a metal shell loses focus for even a second in his navigation through said literal hell.
Then, you finally arrive to the world and go fight against any number of things which want nothing more than to tear you apart and feast on your entrails.
Know what you say to them?
The Emperor Protects.