Pretty much what it sounds like: a system that uses fans to cool down the inside of your computer. Depending on how your computer is configured, it might need enough fans to cool off several different components (e.g. CPU fan, power supply fan, etc.).
When it comes to the guts of your computer, like the CPU, motherboard, and power supply, heat is the enemy. The hotter your computer components get, the slower they run, which makes a good cooling system an essential part of any computer. If you're building your own system you'll need to include a cooling system that'll keep heat levels low. Read on for answers to some essential questions about computer fans and cooling systems.
Fans are the most common type of cooling system you'll find in PCs. Some computers include water cooling systems, which use good ol' H2O to channel excess heat away from the PC components. Older computers used passive cooling systems, but modern systems generate so much heat that active cooling (fans or water) systems are now a necessity.