Exhaust

The exhaust system directs hot, used exhaust fumes out and away from the engine bay, and often muffle the sound of the engine while doing so. The exhaust system can also contain sensors which the ECU can use to improve the engine's performance and reduce the harmful emissions it would otherwise produce.

Not every exhaust system uses the same parts; many race cars don't use mufflers, many older vehicles aren't equipped with oxygen sensors, and the ones that do have those sensors don't always have them in the same location. Because of this, the start-to-finish flow of exhaust gasses can't be described for every vehicle with just one description, but you can find descriptions of each component and their purposes below:

Exhaust Components:

The Typical Modern Order

While older cars did things differently, most modern vehicles have this order of components in common. Think of the flow of exhaust gasses as a stream; upstream is the source of the gasses, the engine, and downstream is where the gasses exit the exhaust routing and mix with the atmosphere, just like a river entering the ocean. Starting from the engine, heading down stream the exhaust is collected into one or two main pipes. The first component encountered will be an oxygen sensor, known as the upstream O² sensor¹. This will take an initial reading of the oxygen in the exhaust. Next to be encountered is the catalytic converter, which uses rare metals to catalyze a chemical reaction with the exhaust gasses. After that will be another oxygen sensor, the downstream O² sensor¹, which is there to make sure the catalytic converter is doing its job, and on some vehicles with extra advanced emissions systems, allow the emissions to be further fine-tuned. After the downstream oxygen sensor will be components not essential to emissions, such as the resonator and muffler, finally ending at the exhaust tip, the only decorative² exhaust component on production vehicles.

Footnotes

1. Changing up the names as we did here, with "Oxygen Sensor" and "O² Sensor" (some even refer to them as lambda sensors with the λ symbol) was not denoting any proper way of saying the main identifier (the secondary identifier, upstream or downstream, is the important distinction), it is only to keep the paragraph from getting too verbally repetitive as well as to not favor one term over the other.
2. Some production vehicles no longer have "real" exhaust tips, and have the actual tip before the bumper so that fake tips can be molded into the bumper and never get exhaust soot on them. Motorcycles and some large trucks may have decorative exhaust components beyond just the tip.