The Alien Campaign is the second campaign available in the Raze series. It is only accessible after you finish the Human campaign. Finishing the campaign in the original Raze will earn the player the "Conqueror of Races" achievement.
The Alien Campaign is generally harder than the human campaign, and the storyline is altered to give the humans (or any side against you) the advantage
The Alien Campaign fills in gaps in the story of the human campaign. It sometimes also shows aliens behind-the-scenes killing, plotting, and generally destroying Earth.