Commissars are men and women who are employed from a young age through cadetships, to become disciplinary officers for the Imperial Guard, the mostly human backbone of the Imperium of Man's arsenal.
They are stern, intelligent and honourable officers who's duty is to search their attached regiment and any others, for threats by the mutant, the genestealer, the heretic. Their word is above that of any member of the Imperial Guard and they will not hesitate to execute anyone that may compromise the safety of the regiment under their watch.
A squad or regiment with an attached commissar on the field is a force to be reckoned with. The Commissar inspires through leadership by leading from the front and through terror by executing deserters who refuse to hold the line or move forward and close in on the enemy. Commissars are people of superhuman resolve, their inhuman tolerance to pain is only through sheer faith in the God Emperor of Mankind. There is no retreat, but there is honour in death.
The feats of an experienced commissar is the stuff of legend in Guard battlefield stories. Such is the true grit of these devoted officers.
Many commissars who have served more than one campaign are heavily scarred, injured and making use of bionic implants and limbs so that they can continue to duty for another day.
There are similarities between a Space Marine chapter Chaplain and a Commissar in that they both provide advice and provide guidance to those they watch over. They seek out the taint and promptly destroy it before it can wreak havoc or even destroy the regiment. Their behaviour and in some cases, fanatical devotion, to the God Emperor of Mankind, wards off the temptation of the warp, the possession of demons and foul deceit that a stressful life in the Guard would harbour, unsupervised.
Colonel Commissar Ibram Gaunt
Commissar Sebastian Yarrick
Contrary to popular belief, a Commissar will not execute a random Guardsman to inspire through terror, as seen in the Dawn of War franchise. According to Warhammer 40,000's official story books, should such an incident come to hand and somebody be executed to bolster the men's resolve, it is likely the soldier executed was causing trouble for the Commissar and the squad mates around him, such as bad attitude, a lack of faith, or somebody holding the squad back by some other means.