From: Viikki Imperium
Haven't you tried a Molotov yourselves? It is not difficult, just some gasoline, burning cloth and a glass bottle. Throw it and watch it burn. As it hits the ground, the gasoline flies everywhere and burns in seconds. The result can be quite devastating for anyone close enough to grab fire.
However, many Molotov cocktails (like those which Finns used in the Winter War) contain far more reactives than just gasolines (petrol etc.) - they can have phospore, sulphure acid, natrium or any highly reactive, burning sample. Thus, they can even burn straighth throug THIN armor (let's say bottom or top) - however, that's rare. Molotovs are not napalm or thermite, they are basicly high-temperature burning liquid in a broken bottle.
Molotovs can, however, affect in many different ways. Blasting the engine is perhaps the most common effect - the burning gasoline will leak through the ventilation hatches and set the engine ablaze. Burning fuel can also leak through vision slots or turret hatch - thus killing crew by heat, monoxide or burns. Molotov can also ignite the oiling in power transfer or other propulsion system parts (I think this is quite rare). And powerful Molotovs can also burn through thin armour. Multiple burning Molotovs in tank's or another vessel's top can cause the temperature to rise into unbearable readings (try finnish sauna and think then how it feels...).
Neverthelss, the Molotovs were no infantry weapons. They were suitable to supress enemy off a bunker, destroying a tank or setting a tent or a pillbox ablaze. However, dug-in infantry should not be afraid of a molotv (31cm Flamer Rocket is a different thing...)
Pain is for the weak.