First of all, this is a very fast 1st person game. (What I meant to say is that you can have rather weak machine to run this game at good qualities and speed.) Missions and the stories are different depending on what species you select. You have three species to select...
Aliens... their armour is quite strong, and they have self-healing possibility. Their only weapons they have are their own body parts... claws or something alike for close combat, their deadly tail, and their inner mouth that can crack through enemy skull is close enough. Their commands are rather bit confusing, 'cuz they can not fall and be harmed. Their floor is where they are, and the wall doesn't make any exception, they climb with ease. They can see the image as a human, or if there is very dark chamber, they can create their own flashlight somehow, that works at a close distance. Their strength lies in flexibility and speed.
Humans... we're talking about U.S. Marines, the one like Michael Biehn acted in movie "Aliens". You can have rather strong firepower that actually ranges from fast-shooting-and-self-locating-target piercing minigun, and flamethrower, all the way to the rocket launchers, and even more powerful weapons. Being a marine is the scariest way to play this game. You have a small radar that makes you nervous showing that someone is near, and you can not see it, whatever it is. You can switch to night-view, only in that case, any light you see will be rather much-anoying. Just remember the scarry animation from "Starcraft" when Zergs (creaturs alike Aliens) were coming a small group of marines. It occurs you that you kill youreself insted, and banish that fear you possess. You can taunt any creature or player around you by screaming or such, but you'll rather get your tail to the other side if some looner alien taunts you with hisssing... :)
Predators... very alike the ones that killed the entire squad of Schwarzenegger's soldiers in the same-named movie. Their power is their strength, and their sight. They have four or maybe more types of seing a picture, self-locating aliens one, self-recognizing humans one, night-vision alike one, and more... They have disks that automatically search for target if in range and sight, only you can hardy see you disks lost in the walls of some battlefield, so you'll have to switch your seing mode to green/light gray one. They have many weapons, and most are rather self locating, destructing, precise or deadly. Predators have the abbility to zoom image in and out if locating someone at far distance.
And the best for last... yes, you can play multiplayer. You can play in several modes, I'll explain only the best ones. You can play all the players against the horde of computer creatures which includes alienators and predaliens that are like stronger mutants and looks rather different, but in that mode of playing, the game is very very difficult 'cuz you can't kill them all, never! Other interesting mode is player against player. You choose the battlefield and the species.