How do you use a graeade in Counter Strike?

You can use grenades to damage, distract, or disorientate enemy players. There are several types of grenades available in Counter Strike. HE Grenades, Smoke grenades, and flashbangs are used while the Decoy grenade, Molotov Cocktail, and Incendiary grenades will be available in Counter Strike Global Offensive.

HE grenades are helpful in flushing out enemy players who are taking cover and provide alternative damage to assist the elimination of exposed enemies.

Smoke grenades supply cover for teammates and help against snipers. It is also possible to throw a smoke grenade at enemies and they may commit friendly fire.

Flashbangs temporarily blind or deafen players who are in the proximity of the flashbang. This allows unaffected players to finish off opponents. Just be warned that if caution is not properly implemented, the grenade may even blind your allies or even yourself and that leaves you very vulnerable for enemy players.

As for the new grenades in Counter Strike Global Offensive, it appears that the Decoy grenade may be helpful in distracting enemy players, intimidate them, or could even supply false suppressive fire.

Finally, the Molotov Cocktail and Incendiary grenades are exclusive to each team (Molotov Cocktails can only be bought by terrorists while the Incendiary grenades are bought by Counter-Terrorists). When used, both grenades ignite flames in an area and damage players who walk through the fire. Unlike the HE grenade, the damage will keep hurting players even if they walked out of the flames (aka after-burn). These grenades are very useful in preventing enemy players from reaching to certain targets (such as a bomb site).

Sources: The Counter Strike Wiki and some experience and analytical information from a Counter Strike player.