I wish there was a demo for Natural Selection 2. It looks really cool, but I know almost nothing about it or how it plays.
It's a fun game but it's got a steep learning curve. Basically NS2 is a mash-up of Counter-Strike and Starcraft. Both factions have commanders who play the game in a top-down rts view while every other player experiences the game as an fps. The commanders direct the flow of battle, they place waypoints, build defensive, support or resource extracting structures as well as research new tech and abilities for the soldiers. In order for commanders to do their jobs effectively or for soldiers to afford to buy upgrades they need resource points. Both factions vie for resource control points on the map. By capturing and holding these points your team and individual players gain resource points. Specific areas on a given map are required to build specific techs and abilities for each faction.
The end goal of a match is to wipe out all of the command centers/hives of the opposing team. The more key areas you hold, the more resources the team and individual players have, the greater the tech/trait advantage, the more likely your team is to win.
The two factions are asymmetrical, the marines have standard fps mechanics but the aliens do not. While the aliens do have some ranged attacks, their primary attacks are melee ranged (bite and claw attacks). Marines can engage the aliens directly with their guns, the aliens (starting form) have to engage the marines somewhat indirectly. You die quickly if a marine can see you coming or you engage a group of marines by yourself. Marines need to stay together in groups, if you go alone you can die easily. Aliens have to climb the walls or use the vents to leap out and surprise attack.
Marines use personal resource points to buy better gear (that the commander has researched) while aliens use personal resource points to evolve to four different lifeforms beyond the initial starting form of the Skulk.
The Skulk is highly mobile, can climb up and around anything. The support class, this hippo thing, can create roadblocks and defensive turret things as well as heal the other classes. It has a decent range attack but you're better off supporting other classes rather than engaging directly. There's a flying class which has a decent range attack (huge energy pool compared to skulks and the hippo) and can cover an area with a toxic cloud (deadly in tight places). There's a stealth attack class which can briefly teleport around a short distance and it has powerful melee attacks. Then there's the Onos, the alien tank class, this crazy alien rhino thing. The alien commander can research traits which improve the abilities of all the forms. You can gain access to cloaking, silent moving, leap attacks, stronger carapace, regenerative health, faster movement speed or a larger energy pool for their abilities. You can only have three traits equipped provided your team holds the necessary map area for them.
Marines can purchase weapons and items like shotguns, welders, jetpacks, flamethrowers, landmines and eventually exo suits (armor suits with gattling gun arms). The commander can also research things like better armor and stronger weapons. The marines are much more involved with the construction than the aliens, the marine commander needs to request things for construction, the marines have to do the building. Beyond the hippo support class the aliens aren't really involved with construction. You just have to ensure that marines don't destroy the creep tumors that allow your commander to build in an area.
There's a tutorial section filled with videos but you really only need to watch one 7 minute video which gives you the basics so you can just hop in and play the fps part.