Space Marines are the elite warriors of the Imperium of Man. They are 7 feet tall super-soldiers enhanced by numerous surgical implants (such as a secondary heart and other neo-organs), extensive medication and hypnotherapy. These enhancements and inhuman amounts of training lead to superhuman physical and mental abilities. Cybernetic interface implants allow the use of their trademark power armour, a heavily armoured suit that augments their strength and provides basic survival/medical functions.
The Space Marines are a small force of extremely skilled and tough individuals. Their veterans have decades of constant warfare under their belt; Dreadnoughts may have centuries. In Dawn of War this "elite few" effect is a bit diluted by the number of troops available - you could lose more men in a single mission than your entire company is supposed to have. Still, you have to admit they're hard as nails.
For context, a sketch of the Imperium is necessary. 10,000 years ago the galactic dominion of mankind had fallen into decline. Colonies were isolated and lost, technology was forgotten; a future dark age. Then on Earth an individual of godlike mental and physical abilities emerged. He promptly united Terra under his rule and set to reclaiming humanity's galactic heritage, becoming the Emperor of Mankind. Somehow the Emperor created a race of übermensch to spearhead the conquest - the first Space Marines, divided into twenty Legions of up to 15,000 men. They were highly successful and shortly (relatively speaking) the Imperium of Man was holding a large part of the galaxy. Yet at the height of their glory almost half of the Space Marine Legions - led by Horus, the Emperor's adopted son and right hand - turned to Chaos and started a civil war. The Traitor Legions sieged Earth and almost destroyed the heart of the Imperium, but on the verge of defeat the Emperor managed to storm the traitor flagship and slay Horus. The Traitor Legions were routed, yet the Emperor was also wounded and placed on his Golden Throne, converted to a life support system, where his withered and silent corpse still sits. 10,000 years later the Imperium is a dystopia locked in endless war with Chaos and numerous alien species. It's an enormous dominion, about 1,000,000 worlds, but spread thin and barely holding together. The whole society is one big war machine where the average citizen works himself to death 20 hours a day on a polluted industrial world or gets eaten by aliens on a frontier world.
And the Space Marines? After the Horus Heresy, the remaining Legions were divided into Chapters, generally consisting of 10 companies of 100 men each. Presently there are about 1,000 individual Chapters meaning a total of about 1,000,000 Space Marines. Each Chapter is a completely self-sufficient force with its own space fleet, support personnel and so on. They are fiercely loyal to the Emperor, but also autonomous and tend to serve as they see fit.
Commanders
Force Commander (Gabriel Angelos)
- Class: Commander
- Role: Melee specialist, air strikes
The Force Commander is your main commander unit, available through every mission. He is an uber-veteran with superior combat skills. He has good ranged damage and great melee damage, and can be upgraded with better weapons and armour. The Force Commander can also order Orbital Bombardment strikes (requires the Orbital Relay structure) which do high damage and knock infantry to the ground. Attached to a squad, he provides a big morale bonus.
In low-level fights, you can send the Force Commander in solo; his melee attack will disrupt infantry squads while your ranged units shoot them up. He will even do decent damage against vehicles and structures. If the enemy has many advanced units, you should be more careful and attach him to a squad. If you lose him, he will respawn around your Stronghold for free after a while, so just use and abuse him.
Librarian (Isador Akios)
- Class: Commander
- Role: Melee specialist, psyker
Inquisitor Toth
- Class: Commander
- Role: Melee specialist, air strikes
Infantry
Servitor
- Class: Infantry
- Role: Builder unit
The Servitor builds your structures and repairs your vehicles, and that's about it. You can set two on the same project to make it happen faster. Servitors take up Squad Cap, so don't keep unnecessary units around. Note that you can Deep Strike them through the Orbital Relay (can be useful if setting up a secondary base).
Scout Marine Squad
- Class: Infantry
- Role: Scouting, sniping
Scout Marines are light infantry that can infiltrate, making them invisible to most enemy units as long as they do not attack. They are lightly armored and have a maximum squad size of 4, so they should generally avoid fights. In single-player they are mainly used to scout enemy positions and spot targets for long-range firepower. When upgraded with sniper rifles they can also take out leader units and destroy enemy morale from behind your lines in bigger scraps. You should get Scout Infiltration Research from the Stronghold right away to make them less vulnerable (besides, it's pretty cheap).
Space Marine Squad
- Class: Heavy infantry
- Role: Versatile
The Space Marine Squad - Tactical Squad in the tabletop game - is your basic infantry unit. They hold their own in both ranged and melee combat; mostly you want them shooting but sometimes charging at ranged specialists. They can be upgraded with a variety of heavy weapons, making them useful in all situations. Upgrading one squad with only one type is best for managing your Marines (i.e., correct range and correct targets).
- Flamers have short range and moderate damage, but will quickly wreck enemy morale, making infantry very ineffective.
- Heavy bolters have long range and great damage against infantry, but cannot be fired while moving and require a short setup time to fire.
- Plasma guns have shorter range but can be fired on the move and are more effective against heavy infantry and commander units.
- Missile launchers have very long range and good damage against vehicles and buildings. They do little damage against infantry, however they will disrupt and demoralize.
There are several global upgrades that apply to Space Marine Squads. You want all, probably starting with the Bionics and Heavy Weapon Increase upgrades. Each squad can be upgraded with a Sergeant who increases squad morale and has the Rally special ability (instant full morale) which should be used a lot. Also get the Frag Grenades upgrade and frag ranged squads to disrupt them.
Assault Marine Squad
- Class: Heavy infantry
- Role: Melee anti-infantry, hit & run anti-vehicle
Assault Marines are jump infantry, meaning they have jetpacks and can be jumped from point A to point B even over impassable terrain. They also move a bit faster than basic Space Marines. Assault Marines are melee units and used most effectively by keeping them back and dropping them over enemy ranged specialists (who should optimally be shot up by your ranged units while engaged in melee). Because of their mobility they can quickly reinforce a weak spot in a fight or prevent an enemy squad from escaping.
Assault Marines can be equipped with short-range melta bombs so they can jump in, bomb a vehicle or building, and jump out. You can also jump them into an enemy base to spot good targets for artillery units (useful if you can't get any Scout Marines in).
The squad can be upgraded with a Sergeant just like a Space Marine Squad. However you cannot attach a commander unit to an Assault Marine Squad, as none of your commanders have jetpacks and they obviously couldn't keep up.
Terminator Squad
- Class: Heavy infantry
- Role: Ranged firepower
These are the most senior veterans of an already elite army, equipped with a heavier version of power armor. The squad can carry 2 vehicle-class heavy weapons; they are slow but fire on the move and also teleport (with a delay). They are excellent - the only drawback is that they take 4 Squad Cap, meaning you could get 2 Tactical Squads (8 heavy weapons) instead, so you must weigh whether they are worth it. In cramped surroundings where it's difficult to fit many squads, definitely - in many other cases, probably not.
Assault Terminator Squad
- Class: Heavy infantry
- Role: Melee specialist
Apothecary
- Class: Heavy infantry
- Role: Healer unit
The Apothecary can be attached to a squad like a commander unit; he doesn't improve the squad's morale or fighting capacity, but boosts the squad's health regeneration (and that of other nearby units). He has a chainsword and can hold his own in melee, but he's not really a combat bonus like a commander. There's no reason not to get and attach the maximum number (4) of Apothecaries.
Vehicles
Rhino
- Class: Vehicle
- Role: Transport
Land Speeder
- Class: Vehicle
- Role: Anti-infantry
Dreadnought
- Class: Vehicle
- Role: Close combat
Hellfire Dreadnought
- Class: Vehicle
- Role: Ranged combat
Whirlwind
- Class: Vehicle
- Role: Missile artillery
Predator
- Class: Vehicle
- Role: Main battle tank
Land Raider
- Class: Vehicle
- Role: Super-heavy tank/transport
Structures
Heavy Bolter Turret
- Class: Building
- Role: Stationary defense
Orbital Relay
- Class: Building
- Role: Deep striking