What does these terms mean rich fuel and raw gas?

it means it,s gas that,s not being burnt.

Internal combustion engines mix air and fuel into a spray and ignite this mixture.The range of this mix varies from about 14:1 up to perhaps 20:1 That is 14 parts of air to one part fuel. Lowering the ratio below 14:1 is a rich mixture, more fuel than normally desired. When an engine runs rich, incomplete combustion might occur, sending unburned fuel or even "raw" gas (w/o air or not in a spray) into the exhaust system, including the catalytic converter. Since the C.C. operates by heat, the unburned fuel products in the C.C. burn and raise the temperature even more so. This shortens the life of the C.C. and can create a "backfire" in the exhaust system; a small explosion of unburned fuel.

Simply put; a rich running engine may seem OK, but can ruin a C.C. very quickly

Overly rich fuel mixtures, particularly at cold engine starts can cause cylinder wall wash down past the piston rings. Raw (liquid) fuel mixes with the oil in the crankcase making it look as if the oil level was too high. The oil looses it's lubricating capacity and high load contact points cause excessive wear.

This is not so apt to happen in fuel injected cars but older cars with bad pull off valves on the carburetors and sticking chokes this phenomenon happens all the time.

your engine runs on a mixture of fuel and air, that is what your fuel injection system does on newer cars and carburetor does on older cars. this mixture has to be a very specific ratio of 14.7 parts air to one part fuel. if there is too much fuel in the mixture the mixture is said to be rich or fat, if there is too little fuel the mixture is said to be lean. raw gas is just hat, unburned fuel that has made it all the way from your gas tank into your engine and then out of your engine and into your exhaust without getting burned. raw fuel, or raw gas is any gas that is still in the state it's in in the gas tank