Emissions are basically chemicals in exhaust gases that are harmful to air quality, mainly carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO). Healthy engines produce fewer of emissions, and older or “unhealthy” engines produce more.
The catalytic converter is used to clean up these harmful gases and expel plain water- when functioning optimally. The emissions system is the biggest cause for a check engine light on all cars and trucks. This was put in place by the federal government many years ago.
There are many, many sensors, valves, solenoids and control system that make up the emissions system. These are now all controlled by the computer, also know as the ECM or PCM.
Some engine designs–like rotary engines, for example–produce more of certain emissions than others and require different emissions controls.