The Smoke Grenade is an incredibly useful grenade that blankets an area with a thick cloud of smoke for 15 seconds. Note that the full effect lasts 18 seconds. Unlike previous versions of CS, smokes in Global Offensive are almost impossible to see through and give a heavy positional advantage to players outside of the smoke. Be very wary of walking through the smoke as enemy players will see you long before you see them.
Tactics & Strategy
Unlike other grenades, Smoke Grenades do not detonate until after they have finished moving, no matter how long they are in the air. As a result, players can throw smokes across very long distances to set up "set piece" plays when attacking or retaking bomb sites. On Mirage, for example, T players with precise positioning can throw a smoke from their spawn so that it lands in the window of Sniper's Nest, effectively negating one of the most common CT defensive positions for the 15 seconds that the smoke grenade lasts. Every map has sneaky smoke throws like this, and against exceptionally well-coordinated teams they are a must.
Smoke grenades can also be used defensively. Most corridors in CS:GO are narrow enough that one smoke grenade, properly placed, will block vision down them entirely. B Tunnels is a good example of this. Because pushing through smoke puts the attackers at a substantial visual disadvantage, this can be an effective way for the CTs (or the Ts, after taking a site) to delay pushes and gain crucial seconds to adjust their defenses as they scout the enemy positions. Using Flashbangs in conjunction with smokes can stall a push entirely.
Occasionally, smokes can also be used to "ninja" defuse a bomb. Because CTs can defuse bombs even if they are standing in smoke, a CT player who doesn't have the time, health, or positioning to fight the Ts head-on will sometimes drop a smoke grenade on top of the bomb and try to defuse it. This tactic is most often seen in 1v1 defuse situations where the T player is hiding and the CT player doesn't have the time or health to track them down before defusing. While the Ts should hear the sound cue of a CT defusing, the lack of vision can delay them for the 5 seconds necessary to get the defuse.