The major alien races are the Orks, Eldar, Dark Eldar, Tyranids, Necrons and the Tau. All these races pose a considerable threat to the survival and continued dominance of mankind. There are countless other races but all are trivial by comparison.
The Imperium considers humanity the rightful rulers of the whole galaxy. Part of the Imperial Creed is that all aliens judged to pose a threat to mankind's survival must be contained or destroyed. Aliens, along with mutants, heretics and deviants, are all considered enemies of mankind, for they threaten the very survival of the human race.