Catalytic converters change harmful substances in a car's exhaust gasses, such as carbon monoxide, nitric oxide, nitrogen dioxide and hydrocarbons, into less harmful substances like carbon dioxide and water vapour by means of chemical reactions.
The interior of the "cat" is usually filled with a honeycomb structure onto which a coating is applied that contains a catalyst - the substance that creates a reaction with the exhaust gasses, changing their chemical structure. Precious metals like palladium, rhodium and platinum are commonly used as the catalyst.
There are various types of catalytic converter. A simple "two-way" oxidation cat works to turn carbon monoxide (CO) to carbon dioxide (CO2) and hydrocarbons, which are basically particles of unburnt fuel, to carbon dioxide and water. More advanced "three-way" catalytic converters are fitted to modern cars and these do the above while also reducing emissions of nitric oxide (NO) and nitrogen dioxide (NO2) which together are more commonly known as NOx, a major cause of localised air pollution.