The smoke grenade is one of the 3 grenades available for humans featured in Counter-Strike: Source. The smoke grenade is almost always an exception among the equipment as it cannot be used (just like the defusal kit and flashbang).
You will never be able to use a smoke grenade on almost all servers. There are some exceptions like Hellz.fr, DreamEvo, servers, who offers the smoke grenade as an exclusive item to VIP's only. On bang, it will freeze all zombies who are touched by its area. Instead of a smoke particle, you will see a blue circle who expands itself for about 1,5 second. Zombies caught in it will be transparent (with a light blue-like color), and all the knockback during the freeze time will be registered, meaning that if you are shot multiple times, you will significantly fly backwards when the effect disappears.