In automotive terms, a misfire is when complete combustion does not occur in the cylinder. The most common misfire is a miss, where the fuel does not ignite and the air-fuel mixture is blown out of the chamber and into the exhaust manifold. The fuel often ignites here due to heat. If it does not, it can frequently flow into the catalytic converter (on vehicles so-equipped) where it may damage the cat by raising temperatures.
The most common cause of a miss is a problem in the ignition system, usually the secondary (high-power) part of the system; the ignition coil, distributor, rotor, spark plugs, or their wires. The other reason would be partial compression of the air-fuel mixture caused by burnt or broken piston rings, valves or valve seats, or damage to the cylinder head or engine block.