Each race has an advantage and disadvantage. The humans have a collection of different units, from heavy weapons experts to medics, and are better known for their long range attacks. Predators are hardcore fighters, taking the battle right up to the enemy, although there are fewer of them available to counter act their superior power. Finally, the Aliens are fast and deadly, and always hunt in large packs.
Upgrade weapons and use terrain to an advantage in each level to make certain your team (whoever or whatever they may be) survive until the next battle. With challenging levels and a variety of objectives, you'll need to stay focused and adapt quickly to succeed.