Warhammer 40,000: Dawn of War/Eldar

(Eldar background)

Farseer
The Farseer is the Eldar primary commander unit. She's a melee fighter with less health than other commanders but more special abilities. Focusing your attacks on her right away (before she can use too much of her abilities) would be smart. She'll die relatively easy because of her low health.
 * Class: Commander
 * Role: Melee, support

Seer Council
The Eldar secondary commander unit is actually a squad of Guardian squad leaders, Warlocks. It's a pretty tough bunch as there are so many and they have commander-class armor and health. They also have some special abilities, though nothing as potent as the Farseer. First and foremost they are the best Eldar melee infantry. You have to kill these before they get to your infantry, or you'll regret it.
 * Class: Commander
 * Role: Melee, support

Guardian Squad

 * Class: Infantry
 * Role: Ranged anti-infantry

Howling Banshee Squad

 * Class: Infantry
 * Role: Melee anti-infantry

Dark Reaper Squad

 * Class: Infantry
 * Role: Ranged anti-infantry

Warp Spider Squad

 * Class: Heavy infantry
 * Role: Jump infantry

Grav Platform - Shuriken Cannon

 * Class: Heavy infantry
 * Role: Anti-infantry

Grav Platform - Brightlance

 * Class: Heavy infantry
 * Role: Anti-vehicle

Falcon Grav Tank
This is basically an armed transport, able to "tank" against an infantry-based force but fried very quickly by proper tanks or anti-vehicle weaponry. Unlike other transports, it can jump like a Land Speeder, which is potentially troublesome.
 * Class: Vehicle
 * Role: Transport, light tank

Vyper
This is a light raiding vehicle, much like a Land Speeder. The Vyper's weapon can be upgraded to an anti-vehicle missile launcher, though. It is fast and has the jump ability. Easily killed when you have vehicles or missile launchers, but possibly nasty if attacking your poorly-equipped infanty early on (like mission 6).
 * Class: Vehicle
 * Role: Fast attack

Wraithlord
The Wraithlord is the Eldar walker vehicle, not as robust as a Dreadnought but more so than most Eldar units. It is most effective in close combat, but also has a shoulder-mounted anti-infantry weapon that can be replaced with a brightlance. It's usually best to avoid and hit with missiles/lascannons, unless you have several Dreadnoughts to beat it down quickly.
 * Class: Vehicle
 * Role: Close combat

Fire Prism
This is evil. Fire Prisms are more durable than other Eldar vehicles (roughly equivalent to Predators) but also fast and capable of jumping. And they have really heavy weapons that do high anti-vehicle damage and blow your infantry all over the place. These are your #1 target for any anti-vehicle weaponry on hand.
 * Class: Vehicle
 * Role: Main battle tank

Avatar
The Avatar of Khaine is the Eldar Relic unit, the ultimate (giant) close-combat fighter. He also boosts their Squad/Vehicle Caps, production times and morale. However, as you only see him in mission 7 where he raids your base, you would be mostly concerned about the close combat part. A Dreadnought to challenge him (or dancing if you don't have one) with concentrated fire from everything else should do the trick.
 * Class: Daemon
 * Role: Close combat, support/infrastructure