The Genestealers are a race of aliens who seek to conquer the universe. They travel in huge, nearly derelict vessels, commonly known as space hulks, until they find a suitable planet to 'colonize'.
A victim of the Genestealers truly suffers a fate worse than death. Injected with the Stealer's egg, the victim becomes an unknowing carrier of the Stealer's DNA pattern, and his children are born as Genestealers. These creatures, in turn, infect other humans, until the entire planet is taken over.
The Emperor of Human Space has recently become aware of the Stealers' actvities and is taking immediate steps to isolate and stamp out the attack - but at a terrible price - the Emperor's Inquisitors ruthlessly sterilize tainted planets, wiping out their populations to the last man, woman and child.
Though thus far 100% effective in stopping the spread of the Stealers beyond the infected planets, this solution is not without its drawbacks. In addition to the obvious loss of life and resources, if news of the scorched earth policy spreads, local governments may naturally become reticent about telling the Imperium that they have been infiltrated, making the job of crushing the invasion all that much more difficult. Therefore, great efforts are being made to search out the invading vessels and attack and destroy them before they have a chance to infect any more planets.
Enter the Space Marines.