From Exterminatus Manual
Exterminatus focuses on two of the many races from the Warhammer 40,000 universe, the Imperium of man and an alien race known as the Tyranids.
In the 40th millennium humanity has colonized much of our galaxy. Millions of human worlds are ruled over from Terra (Earth) by the Emperor. Each of these worlds contribute a regiment of troops to the army of the empire, the Imperial Guard. They are the hammer with which the Imperium smites its xeno (alien) foes beyond the empire and rebellions within it. The Imperium has a second, smaller fighting arm, better equipped, trained and physically augmented, the Adeptus Astartes, the Space Marines. There are a thousand chapters of marines within the empire, each with a thousand marines and yet they number less than one marine per human world. They are the scalpel to the Guard's hammer.
Named after the planet Tyranus where they were first discovered, the Tyranids are an alien threat like no other. They have traveled from the cold, dark abyss between galaxies. They are a voracious horde of beasts, unified by a single conciousness, the 'Hive' mind. All of the Tyranids technology is organic: their ships with which they strip planets to the bedrock, the weapons with which they slay all before them. They know no fear, they cannot be argued with, they cannot be reasoned with and they absolutely will not stop. Wave after wave of Tyranid 'hive fleets' have been broken on the galactic disc, each ravaging further, pillaging more worlds than the last.
Class costs increase from left to right. The left most class on each team is effectively free, with the remainder being upgrades offered for playing well.