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

Chaos Lord (Bale)
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.
 * Class: Commander
 * Role: Melee, support

Chaos Sorcerer (Sindri)
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.
 * Class: Commander
 * Role: Support

Heretic
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.
 * Class: Infantry
 * Role: Builder unit

Cultist Squad
Cultists are weak and numerous, a bit like Imperial Guardsmen except more effective in melee than ranged combat (unless equipped with heavy weapons). Cultist Squads may have grenade launchers or plasma guns just like Guardsmen. Their squad leader (Aspiring Champion) can detect infiltrated units and can be used for the Bloodthirster transformation. It is best to mow them down with heavy bolter fire before they tie you up in melee. A Land Speeder will make short work of Cultists.
 * Class: Infantry
 * Role: Anti-infantry, cannon fodder, detection

Chaos Space Marines
Chaos Space Marines are pretty much just like your (Tactical) Space Marines. They have roughly the same abilities and can be upgraded with the same heavy weapons (flamer, heavy bolter, plasma gun, missile launcher). You should check which heavy weapons (if any) a squad has before deciding who to take out first - especially watch out for missile launchers targeting your vehicles. Your infantry is most effective with plasma guns; also consider using Assault Marines or Deep Strike if a squad is hurting you from distance.
 * Class: Heavy infantry
 * Role: Versatile

Raptor Squad
Close combat unit capable of short distance flying.
 * Class: Heavy infantry
 * Role: Jump infantry

Horror Squad
These are minor daemons of Tzeench that can Deep Strike from the Sacrificial Circle building. They come in squads of 5 and have a potent ranged attack against infantry. They are ineffective against vehicles (unlike in the expansions) and quite useless in melee. Thus you should obviously try to tie them up in close combat if possible.
 * Class: Infantry
 * Role: Anti-infantry

Possessed Squad
"The ultimate combat unit" with a variety of upgrades.
 * Class: Heavy infantry
 * Role: Melee

Obliterator Squad
A squad of pure destruction that can lay waste to forces by using a variety of weaponry.
 * Class: Heavy infantry
 * Role: Ranged firepower

Chaos Rhino Transport
This is just like the Space Marine transport, only with spikes. The enemy doesn't seem to use these much in the campaign.
 * Class: Vehicle
 * Role: Transport

Defiler
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.
 * Class: Vehicle
 * Role: Versatile

Chaos Predator
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).
 * Class: Vehicle
 * Role: Main battle tank

Bloodthirster
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.
 * Class: Daemon
 * Role: Melee

Daemon Prince
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.
 * Class: Daemon
 * Role: Melee