First, there’s a multitude of factions, ranging from the the Imperial Guard and Space Marines, to the Sisters of Battle and the Inquisition. Chaos Space Marines, Eldar and Dark Eldar, the Tau Empire… to name some.
With the essentials out of the way, I’m going to focus on the Imperial Guard, or Astra Militarum as Games Workshop has renamed them.
With hundreds of planets in the Imperium, it can be said that a handful of notable regiments stem from the vast millions. The Cadian Shock Trooper and Catachan Jungle Fighters, as well as the Tanith First and Only, as made popular by the Gaunt’s Ghost series of books, are only the tiniest handful to be named. Each regiment varies in strength. Some only number in the hundreds at their founding, while others will number in the tens of thousands. In the digital world, you will find the Guardsmen of Cadia in several of the Dawn of War titles as well as in Warhammer 40K: Space Marine.
Like any real world military, there is an absolute command structure with a Colonel often being at the highest rank of command. Within each Guardsman regiment is attached a Commissar, making the military structure something similar to that of World War 2 era Russia. These political officers function as moral boosters even though that, more often than not, moral is boosted by the terror of being shot for cowardice..
Further, regiments are then formed into a variation of types. From the heavily armored tank regiments to the more lightly equipped light infantry regiments.
A running joke within the Warhammer community, because of the Guardsman’s standard issue Lasgun being nothing more than glorified flashlights, is that ‘when in doubt, fix bayonets’.
Taking that in, a person using an Astra Militarum army for the tabletop game should never employ such tactics as, without armored support… Guardsmen are just so squishy, or in other words, easily overwhelmed by a superior force. Especially against Space Marines and their chaotic counterparts.