CPU Cooler
A CPU cooler is a computer component used to dissipate heat generated by a CPU under computation load. This is required as the cooler a CPU operates, the faster it will run. Running a CPU too hot can also cause irreversible damage to the component.
Air Coolers
The most common and cheapest type of CPU cooler are air coolers. In an air cooler, heat pipes contact with the top of the processor and dissipate heat via many fins attached to the pipes. Most air coolers have fans attached to them to blow hot air away from the fins, and those that do not are called passive coolers.
Some processors (e.g. Ryzen 3/5/7's, None-K Intel i3/i5/i7's) come shipped with an air cooler, referred generally as a stock cooler. Third party air coolers typically perform better then their stock equivalents.
AIO Water Coolers
Custom Water Cooling
Thermal Paste
FAQ
Should I buy a third party CPU cooler, or is the stock cooler good enough?
Should I get a third party air cooler or AIO water cooler?
Are custom water cooling loops worth it?