Marks of Chaos
Khorne - Chaos Armour, savings roll of 4+, may be combined with shield and/or cavalry bonus. Frenzy, Chaos Champions of Khorne are affected by the rules for frenzy as described in the Warhammer rulebook.
Tzeentch - Magic Dispel, 4+ on a d6.
Nurgle - Toughness +1 to Toughness.
Slaanesh - Psychology, Champions of Slaanesh are not affected by any of the psychology rules. Combat, Champions of Slaanesh do not take break tests, if fighting as part of a unit they do not flee if the unit flees, instead they are immediately separated from the unit so that they can continue to fight on.
Chaos Rewards
Chaos Rewards are special abilites that may be purchased for Chaos Champions and/or Sorcerers as per the rules in Codex: Chaos.
Chaos Wargear
Chaos Marines are generally limited to those weapons available before and during the Horus Heresy. Many of the weapons in general use by the loyal Space Marine Chapters of the 41st millenium did not exist or were only experimental at the time of Horus's rebellion. This is reflected in the use of Mk I plasma pistols and guns, and the fact that the Mk I heavy plasma gun can only be mounted on a Dreadnought. It is also reflected in the lack of such things as Hellfire shells. They also have a lack of targeters because the targeters of the day were bulky and prone to malfunction, so most heavy weapons were not fitted with them.
On the other hand, Space Marine Legions at the time of The Heresy used a more diverse variety of assault weapons, including power maces and chainaxes, which have become virtually unknown in the Imperium of the 41st millenium. Power swords were an exception to this, as the secrets of their manufacture were still lost, making any that were uncovered rare and valued artifacts. Weaponry used by Terminator squads was also very different during The Heresy than they are in the loyal chapters of the 41st millenium. Traitor Terminator armour lacks such things as targeters and teleport homers, either because the armour is too old to include them or because they have malfunctioned and never been repaired.
Jump packs ans skimmers were in limited quantities at the time of the Heresy and they have been eliminated from the Traitor Legions due to their lack of capability to keep them repaired.
Terminator Weapons - Traitor Legions do not have access to storm bolters. Instead they use combi-weapons of twin bolter, bolter-flamer, and bolter-melta (bolter-plasma was very rare and was not issued to squads). The only combi-weapon available to them that can fire both barrels at once is the twin-bolter.
Reaper Autocannon - One of the most successful early weapons for Terminator armour was the reaper autocannon, which was later developed into the assault cannon. The Traitor Terminators, lacking the technology and equipment to maintain the fickle assault cannon, still make use of the reaper, with its simpler mechanism and solid ammunition.
One special weapon that needs mentioning is the Blight Grenades used by The Plague Marines. These are containers of pestulence and biting flies that is stoppered with wax and splatters across the target area when it is thrown, forming puddles of purulent slime and noisome clouds of buzzing flies.
Other special weapons are the weapons of the Noise Marines. These use wailing unearthly music as weapons.
Chaos Dreadnoughts - These great machines contain the trapped body and soul of a living Chaos Marine. His millenia of entrapment inside the machine has invariably driven him mad. The Chaos Dreadnoughts are kept in a state of stasis when they are not fighting in the wars of their Legions. The insanity of the Chaos Space Marine trapped in the Dreadnoughts is represented by the Dreadnought being subject to both the rules for frenzy and stupidity. Because the frenzy and stupidity rules can't both apply at the same time, you must use the procedure on page 77 of Codex: Chaos.
Chaos Icons
The Chaos Icons are very important to the Chaos army, and should be chosen with care, thinking of the force that you will be using and the mission that you have set for it. The icon will affect the whole theme of your Chaos Marine army.