Exhaust gasses are the result of the combustion of fuel and air. They leave the engine though the exhaust system. Some of these gasses are considered pollutants, and are altered by the catalytic converter if the exhaust system has one.
The majority of most types of combustion gasses are nitrogen (N2), water vapor (H2O) (with the exception of pure-carbon fuels), and carbon dioxide (CO2) (except for fuels without carbon in them). Carbon dioxide is generally referred to as a greenhouse gas that contributes to the process of global warming.
A moderately small part of combustion gas is undesirable noxious/toxic substances, such as carbon monoxide (CO) which comes from incomplete combustion, hydrocarbons (indicated as CxHy) from unburnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, Ozone (O3), and particulate materials (mostly soot).