Warhammer 40,000: Dawn of War/Orks

The Orks - also known as Greenskins or Boyz - are a weird alien species of unknown origin. It is speculated they were created as bodyguards by some extinct race. They are somewhat larger than humans, very tough and very aggressive; if there's nobody else to fight, they will fight each other. The only things they care about are fighting and going fast. Orks thrive on conflict and tend to keep getting bigger the more they fight. The biggest Ork always leads the group; if another Ork gets even bigger, he'll kill the old leader and take over.

Biologically the Orks are a strange animal-fungal hybrid that reproduces by wind-borne spores (yes, seriously). This makes them extremely hard to eradicate, and grants them exceptional toughness and regenerative ability. They can heal after dismemberment or even decapitation if the missing part is just put back in soon enough.

Ork infantry has huge numbers and their units are mostly melee-oriented. Their technology is crude but functional, and combined with their crazy aggression it seems to hold up against the more advanced races. They have a habit of scavenging and modifying enemy vehicles, an example being the Looted Leman Russ Tank.

Warboss (Orkamungus)
The Warboss is the primary Ork commander. His attacks (against anything) really hurt, he can take tons of damage, and he can improve the performance of nearby infantry. When you have to fight him (in mission four), you don't have any of your top-tier units, so you'll just have to concentrate all fire on him. It may still take a while and you could take a casualty or two.
 * Class: Commander
 * Role: Melee

Big Mek

 * Class: Commander
 * Role: Melee

Gretchin Squad
Gretchins are the Ork builders. The have minimal fighting ability and aren't really a threat to take seriously, but in the early missions some will attack you anyway. They build really fast, however, and if you're destroying a camp with a small force it may seem like they repair the buildings faster than you can damage them, or build new ones instead.
 * Class: Infantry
 * Role: Builder unit, nuisance

Slugga Boy Squad
Slugga Boyz are the Orks' basic infantry unit. They are cheap and numerous (up to 15 per squad) and can be upgraded with flamers. Try not to get tied up with them, shoot them instead. Any heavy weapons will do, but heavy bolters would be best as Slugga Boyz have light armor and you can stay far away.
 * Class: Infantry
 * Role: Melee anti-infantry

Shoota Boy Squad
Shoota Boyz are a bit more advanced, being equipped with shootas and upgradeable with big shootas. If they have regular shootas, you can just shoot them, they are no match for Space Marine heavy weapons. If there are many big shootas, though, you may want to melee them (esp. with Assault Marines) to avoid casualties.
 * Class: Infantry
 * Role: Ranged anti-infantry

Stormboyz

 * Class: Infantry
 * Role: Jump infantry

Stormboyz are the Ork equivalent of Assault Marines. They will jump on your ranged infantry (or artillery, which isn't a problem as they have poor damage against vehicles and no melta bombs), so you may have to dance a bit. Having a flamer squad waiting by when they land will make short work of them, however.

TankBustaz Squad

 * Class: Infantry
 * Role: Ranged anti-vehicle

Nob Squad

 * Class: Heavy infantry
 * Role: Melee specialist

Mad Dok
The Mad Dok is the Ork equivalent of your Apothecary, only with better fighting capacity. He has a Fightin' Juice special ability that works like your Word of the Emperor, preventing affected Orks from dying. Try to pick him out with plasma guns or the like.
 * Class:
 * Role: Healer unit

Wartrukk

 * Class: Vehicle
 * Role: Transport, anti-infantry

Wartrak

 * Class: Vehicle
 * Role: Anti-vehicle (anti-infantry)

Killa Kan

 * Class: Vehicle
 * Role: Close combat

Looted Leman Russ Tank
This is a pimped Leman Russ stolen from the Imperial Guard. There's no real difference to the Guard version; both have the battle cannon and several heavy bolters. These can take a lot of punishment and be a real nuisance if there's many. Try to take them one at a time, as in a 1:1 shootout they will lose to a simple missile squad (due to their inaccuracy and you staying out of bolter range).
 * Class: Vehicle
 * Role: Main battle tank

Squiggoth

 * Class: Vehicle/daemon
 * Role: Super-heavy weapons platform & transport