I never tried steam b4 and i am just wonderin , its better to play mutliplayer as in crysis 2 on steam ? and are steam players can play crysis 2 with other people that not activated by steam ?

just got no idea about steam :S

the game will use steam to launch n keep it files in the steam folder it thats about it
you can play with everyone who has crysis2

I do believe Crysis 2's MP servers are being runed by EA, so even if you buy it off of Steam or in a store or what have you, you can play with other players on PC, I don't know if there's cross platform MP game play, but if there is, then yeah.

Anybody who purchases the game for the PC platform can play with each other, the retailer or digital download service you purchase from will not affect the multiplayer experience.