"You are a human, these are your friends, and one of them is going to kill you. As a human you have two goals. The first and most important is to survive until extraction. The second is to find and kill all the Brood Aliens. If you survive until extraction or you and your fellow humans kill all Brood Aliens then you will win. Alone you will be picked off one by one and eventually lose. Together you run the risk of having a Brood Alien in your group, which can be just as dangerous. Choose your friends, and keep them close, but not too close.
You are an alien, this is your prey and you must deceive them and strike when their guard's down. As a Brood Alien you are able to disguise yourself as a human to deceive the humans into thinking you are a human. At any time you are able to switch into your true alien form and attack! If you kill someone while in your alien form they will become a Brood Alien just like you. Your goal is to infect or kill all humans."