Imperial Guard are the forefront of the Imperium. With inexaustible platoons of Guardsmen and heavy tanks backing them up, they form a very formidable force on the battlefield.
Some players prefer them because of their close representation of the human race.
They're infamous for their lousy weapons, aim and armor.
They often arrive in hundreds on the battlefield, and it's very common for them to be shred to pieces by whatever gets in their way.
They're loyal to the emperor and come from many diverse planets. The planet they come from often defines their look. Some example regiments are Cadians, Catachan and the Valhallans.
The Imperial Guard is as close to cannon fodder as you can get.