A copper or metal device designed to dissipate heat from whatever it touches.
Some heatsinks come with fans
Product Types
Original Equipment Manufacturer (OEM) CPU Coolers
These are CPU manufacturer-made heatsinks specifically designed to be compatible with your CPU.
Buying some of these ensure that your CPU is 100% compatible
Specialized CPU Coolers
3rd party heatsinks that usually exceed OEM coolers in the ammount of heat they dissipate.
Installation
Many heat sinks will have a thermal pad at the bottom. If there is tape, remove that tape and then apply the heat sink over the CPU after Thermal Paste is applied.
Intel
Applying the heat sink can include installing 4 plungers
AMD
AMD will commonly require you to secure 2 points outside the CPU socket and lock down a latch