It is a weapon which can be used to set ablaze a target. It's a bottle containing inflammable substances like petrol, diesel, kerosene, etc which can be ignited with the help of a cloth wick. It burns instantly and is used by those fighters who don't have enough money to buy Hand Grenades.
It is available in Grand Theft Auto. It is introduced during the challenge of Burning Desire. Also, it was there in GTA Vice City and GTA Chinatown Wars.
How To Throw/Use a Molotov?
You can throw it by pressing the button which you use to shoot guns while playing GTA SA on PC or Xbox or PS2. Thus, on PC, use Left Mouse Button (LMB) to throw it. For throwing it over a larger distance, hold the LMB for longer time. Thus, to increase distance hold down LMB for longer time.
Cheat For Molotov:
If you want it any time then you can use the cheat LXGIWYL for getting it. And then use the it for infinite ammo which is FULLCLIP and thus you have infinite Molotov.
PS2 cheat for it is: R1, R2, L1, R2, Left, Down, Right, Up, Left, Down, Right, Up.
Xbox cheat for it is : R1, R2, L1, R2, Left, Down, Right, Up, Left, Down, Right, Up.