Warhammer 40,000: Dawn of War/Chaos Space Marines

The primary commander unit for Chaos, the Lord is a melee specialist who will usually beat any of your commanders one-on-one but is easily defeated when you concentrate on him with other nearby troops (you should have a melee unit to engage him so he doesn't get to your ranged infantry). He has a morale bonus and can be used for the Bloodthirster transformation - although in the campaign Bale will be gone before you first encounter the daemon. When fighting the Chaos Lord, you can consider how you would counter your own Force Commander.

Chaos Sorcerer (Sindri)

Class: Commander
Role: Support

The Sorcerer is the secondary commander, the Chaos equivalent of your Librarian, although his special abilities are more offensive than the Librarian's. The Sorcerer has decent melee ability but lower health than his peers, so he's more a "spellcaster" than a front-line fighter. His abilities cause damage and movement restriction to enemy units instead of buffing friendly units. He can be used for the Bloodthirster transformation.

Infantry

Heretic

Class: Infantry
Role: Builder unit

The basic builder/repairman unit, weak and fragile. However the Heretic can activate a special ability called Forced Labour that allows him to work really fast - while constantly losing health, potentially killing him. A good ability when used with skill.

Cultist Squad

Class: Infantry
Role: Anti-infantry, cannon fodder

Weak unit with high numbers useful as a meat shield. Can be upgraded to detect infiltrated units, have plasma guns and Grenade launches.

Chaos Space Marines

Class: Heavy infantry
Role: Versatile

Similar to standard Space Marines however tainted by Chaos. Can be upgraded to have heavy bolters and plasma guns.

Raptor Squad

Class: Heavy infantry
Role: Jump infantry

Close combat unit capable of short distance flying.

Horror Squad

Class: Infantry
Role: Anti-infantry

Ranged attack daemon summoned from the warp into the map.

Possessed Squad

Class: Heavy infantry
Role: Melee

"The ultimate combat unit" with a variety of upgrades.

Obliterator Squad

Class: Heavy infantry
Role: Ranged firepower

A squad of pure destruction that can lay waste to forces by using a variety of weaponry.

Vehicles

Chaos Rhino Transport

Class: Vehicle
Role: Transport

This is just like the Space Marine transport, only with spikes. The enemy doesn't seem to use these much in the campaign.

Defiler

Class: Vehicle
Role: Versatile

A daemon-possessed, four-legged walker vehicle. The Defiler has anti-infantry weaponry, a strong melee attack and an (inaccurate) battle cannon that will disrupt/demoralize infantry and deal decent damage to vehicles and buildings - a "Jack of all trades". It's best to keep your distance and take it out with lascannon vehicles - a missile squad will do, but may get knocked around a bit before succeeding.

Chaos Predator

Class: Vehicle
Role: Main battle tank

Equivalent to the Space Marine Predator. Starts out with anti-infantry weapons and can be upgraded to lascannons. Pretty durable so it needs some focused anti-vehicle fire (or Dreadnoughts if you can get them close enough).

Daemons

Bloodthirster

Class: Daemon
Role: Melee

This is the Relic unit for Chaos. It's a greater daemon who's a dedicated (giant) melee fighter with jump ability. The Bloodthirster is built from the Daemon Pit, but doesn't actually appear there. Instead he's activated from a commander or squad leader unit who is then killed and replaced by the Bloodthirster. Hence one of these could show up unexpectedly in the middle of a fight. Killing one isn't much different from killing an Avatar; you dance with, say, an Assault Squad while focused fire wears it down. Your commanders can hurt it pretty good as well.

Daemon Prince

Class: Daemon
Role: Melee

The ultimate Chaos unit, a hero of Chaos "ascended" to a position second only to the Chaos gods. Treat him like a tougher version of the Bloodthirster. Of course, killing the Daemon Prince will finish the whole game so feel free to mob him with all your melee units and keep dropping Dreadnoughts on him as fast as you can build new ones.