oh yes, each 'level' of alien has their own 'perks'
but for what i think you're looking for is the basic one 'skulk' (you spawn as this) has the ability to crawl on walls and ceilings, mainly for gerrilia warfare. it can shoot 'parasites' where it makes that marine, or structure visual to all the rest of the aliens, so that everyone knows where that is (until they die of course)
the gorge is the 'builder', make structres that help out give abilities to the team, and make more 'hives' (up to three per map, which give more abilities on completion)
lerk's fly and spit out acid clouds that eat up the marines
fades zip in smack marines, and zip out, heal then come back.
then the 'onos', they're big, mean, and eat other players whole. (literally, and they get 'crapped out' XD)
the marines side get weapons and armour upgrades, jet packs, heavy armour. drop turrets, get motion sensors and the like.
the only thing about the marine weaponry, it doesnt use the CS style of projectile aiming, its more like, spray and pray, type. but you get a bit of weaponry to use: LMG's, HMG's, SG's, Gren Launchers

, and your trusty pistol, and if needed, a combat knife (which if you'd use mainly out of desperation cus of reload times...

)