Natural Selection
Natural Selection is also called NS. NS is a game where you must destroy & capture opposite's every place and character. If you are marine, you must destroy their hives and kill last of aliens. If you are alien, you must destroy every marines IP's those who will "reforce" them. NS is modification of Half-Life. NS is free after you buy Half-Life game.

Team - Aliens
Aliens are much different than marines, Aliens have many forms, but marines actually have just one form. Aliens duty is to capture all hives and resource towers, if you transform into gorge and "join" creating Offense Chambers, Defence Chambers, Sensory Chambers, Movement Chambers, Hives, Resource Towers it will be very helpful to your teammates. When you have captured all Resource Towers and hives, the marines are devoid finish, the easiest way to beat them is that everyone in your team will transform into Onos and rush into marines last base.

Chambers
Offense Chamber - Offense Chamber is very good "aliens shooting brick", to create this OC you must be in the gorge form and have at least 9 resource. Mostly its very useful to create "a wall" with Offense chambers.

Defense Chamber - Defense Chamber is known as DC, its awful looking "creature" which grant health to every chambers and players. Generally its good to create Defense Chambers to near hive, Offense chambers behind or wherever you want to put it. This "cost" 8-10 resource.

Sensory Chamber - Sensory Chamber will completely hide the area that its been created. Marines cant see any of chambers or hives or resource towers or even aliens if they dont hit. The aliens are still able to be hurted or killed. There is many ways to avoid this little "trick" one way is that commander is going to scan that area.

Movement Chamber - This Chamber is like "teleport", it will teleport you to hive location or to another movement chamber.

Hive - You can create many MC's,SC's,OC's,DC's, but you can create only three hive. Hives are mostly important "things" to aliens, Hive is place where you are going to spawn, if marines will succesfuly destroy all hives, alien are going to lose.

Team - Marines
Marines are humans, those who are going to attack aliens. The marines main object is to destroy all hives and capture their hives, also Resource Towers. Alien team have system where all aliens can build, but marines have just one builder: Commander. Marines can use few different weapons and armors and many other "little stuff".

Commander
Commander is most "respected" player in the marine team if he is good. Commander will locate all the "buildings" that he will build, even weapons, mines, armors. Usually if you are playing Natural Selection NS_ map, you can see Commander sitting down in the CC = Commander Chair. If you want to be a commander, you should be one of the first in the marine team and run into commander chair as fast you can.

Download
Do you have Half-Life game and you are inrested about Natural-Selection? No problem, i give some mirrors where you can download NS. Google is your friend too. (www.google.com)
Link 1

Pictures
There is no pictures added by me, but by NS.org.
Marine:| Player | Weapons | Structures |
Alien:| Player | Weapons | Structures |
Currently Pictures by www.natural-selection.org