The Imperial Guard are the defenders of the universe. While not as powerful as the Space Marines, the Imperial Guard is by far more numerous. There is a legion of Imperial Guard on almost every planet that there are humans. If there is ever a battle to be fought, the Imperial Guard are the first ones to be called.
The Imperial Guard recruits from almost every planet in the Imperium. Some of the
planets that
have the largest legions are Cadia, Mordia, Catachan, Tallarn, Valhalla, and Attila. The Cadian shock troopers are very tough, grim soldiers, because there planet is located so close to the Eye of Terror. They are constantly fighting the forces of Chaos. The Mordian iron guard are also very grim and determined, and are in fairly close proximity to the Eye. The Catachan jungle fighters
come from a world where only the toughest survive. There are plants and beasts on Catachan everywhere that can kill a man.
The Tallarn desert raiders live in a harsh desert environment. The Vinhallans come form an ice world, where if you don't stay warm, you die. The roughriders of Atilla travel mostly on horseback. The Imperial Guard also recruit ogryns, giant mutants perfectly suited for fighting. They also have units of Rattling
snipers, dwarf mutants known for their marksmanship.
Imperial Guard armies are called regiments. A regiment is led by a colonel, and is made up of companies. Each company is led by a captain, and is composed of up to 6 infantry platoons. Each platoon is led by a lieutenant, and is made up of five squads of men.
For HQ, you take a command HQ, either led by a captain, or a colonel, and comprised of four additional guardsmen. A single HQ choice may also have up to five heavy weapon squads, (each squad has three heavy weapon teams). There can also be up to five commissars in an Imperial Guard company. Commissars are incredibly loyal to The Emperor, and will not hesitate to punish any commander who fails him.

F.
or troops, the Imperial Guard have platoons. There are up to six platoons in a company A platoon is led by a command section, consisting of a lieutenant and four guardsmen. There can be up to five infantry squads in a platoon. The troops can be mounted in a Chimera transport tank.

For elites, there are hardened veteran squads, storm trooper squads, ogryns, and ratling snipers. Hardened veterans are just veteran guardsmen. Storm troopers are the most elite, most advanced guardsmen there are. They get very advanced weapons and armor, and they can infiltrate or para-drop to get the jump on the enemy. The ogryns are huge mutants, which while primitive, make excellent fighters. The ratling snipers are the marksmen and the cooks for many Imperial Guard armies.
For fast attack, they have Rough Riders squads, and some light vehicles. One is the Hellhound, which is based off of the Chimera chassis. It can't transport anyone, but it has a powerful flame weapon. There are also Sentinel squadrons, light scout walkers.
For heavy support, the Imperial Guard probably have the most. They have a bunch of powerful tanks. The ones based off of the Chimera chassis are the Basilisk and the Griffon, and lots of other, less popular ones. The Basilisk has a massive artillery cannon, and the Griffon has a heavy mortar. Then there are all the variants of the Leman Russ battle tank, like the Leman Russ Vanquisher, the Leman Russ Exterminator, and the Leman Russ Demolisher, as well as many others.