As air pollution became a political football in the 1970s and 1980s, politicians looked to the car makers to cut down on emissions.
The result was the catalytic converter, part of the exhaust system that heats the exhaust fumes to extreme temperatures, turning them into inert gases.
It acts like an afterburner that burns up any gases that remain unburned in the exhaust system. The three main emissions it's designed to reduce are carbon monoxide (CO) - a poisonous colourless gas, hydrocarbons or volatile organic compounds (VOCs) - produced mostly from unburned fuel that evaporates, and nitrogen oxides (NOx) which contributes to smog and acid rain, and also causes irritation to human mucus membranes.
Today most modern cars are equipped with three-way catalytic converters, to deal with these three emissions.
There are two main types of structures used in catalytic converters - honeycomb and ceramic beads. Most cars today use a honeycomb structure.
Older cars with catalytic converters tended to lose horsepower and fuel efficiency.
This caused many people to remove them (illegally) in order to get the power back. Newer cars have revised converters which make removal pointless for street-use.