Commander / Void Commander
The Rank of Commander is the highest point at which a soldier can serve his or her Dread Lord. Each Dread Lord is permitted three of each though exceptions can be made at behest of , Commanders and Void Commanders. The former are responsible for ground combat and anything relating to planetary defense from the surface, the latter are often given command of battlegroups, both in the attack and defense of the Dread Lord's holdings. Commander's and Void Commanders hold the same technical rank, though it is unlikely one would ever be put in command of the other's forces - each to their own specialty. As can likely be guessed, Commanders are in charge of ground forces, Void Commanders are are in charge of the fleets and space-craft of their respective Dread Lord.
Captain
Captain's are the backbone of the force, leading not only soldier into battle but also commanding vessels of the void. On the ground, Captains are put in charge of large numbers of forces, typically up to several Hordes - Hordes consist of 300 soldiers, broken down into groups of 50 called Cabals (Subject to Change). Each Horde is led by a Lieutenant and then each Cabal within the horde is led by our next subsequent rank - the Sergeant.
Void Captain
Void Captain is a notable rank amidst the armed forces of the Dread Lords, each in command of a vessel of the void. They are responsible for not only carrying out the orders of their Lord but maintaining and caring for their crew. Some are harder than others, some smarter, but at the end of the day they get the job done. Upon a Void Lieutenant's promotion to Captain, he is awarded a vessel of his choice which is then commissioned under their respective Dread Lord's forces.
Lieutenant
A Lieutenant is subordinate to the Rank of Captain and put in charge of a 'Horde' of soldiers, numbering 300. The lowest officer in the Army of Chaos (The collaborative forces of all the Chaos Lords), Lieutenants are responsible for coordinating and commanding their hordes on the field of battle. Each Horde is divided into six 'Cabals' of fifty soldiers.
Void Lieutenant
Void Lieutenants, unlike their army counterpart are exclusively stationed aboard the naval vessels of their Dread Lord's fleets. Typically assigned key positions aboard vessels, they can also be tasked to lead expeditionary parties to the surface in situations where the conventional ground forces are unavailable. Even so, they are no less formidable.
Sergeant
One of the most important ranks in the forces of the Chaos Lords, these soldiers are veterans of warfare. Driving their men to do great and horrible things in the names of their Lords, these men instill a sense of fear and respect amongst their men, always driving themselves and those around them higher and farther. Sergeants are placed in charge of a 'Cabal' numbering 50 soldiers. Sergeants stationed aboard vessels are referred to as Void Sergeants.
Soldier
The very flesh and blood that makes up the bulk of the endless hordes of the Chaos Lords. Born and bred into warfare, they learn quickly that might is right. These soldiers fight for their Lords as if they were gods. Death doesn't faze them, threat of the void won't dissuade them. They are ready to die - for glory, or for their Lord and their King. Void Soldiers are of the same ilk, though stationed aboard their Dread Lord's vessels.
Thrall
The lowest of the low, these beings are those just strong enough to scrape by the bottom of the barrel - and they do so by serving those stronger than they. The filth of the Hordes, these beings perform the most menial of tasks, cleaning armor, scrubbing the garbage chutes - you get the idea. Though they are able to fight along side those they serve, their weapons and gear are entirely scavenged from the battlefield. Survival is in their nature.