CPU is short for Central Processing Unit. When someone writes a program such as Microsoft Word, Adobe Photoshop, or Quicken, they're actually writing instructions to be carried out by your computer's CPU. The faster your CPU, the faster the program's instructions can be processed. The CPU generates a lot of heat when the computer is turned on. A large piece of metal, the heat sink, sits on top of the CPU and draws heat out of it. The fan blows air over the heat sink to further cool it. The speed of the fan on top of the heat sink is usually controlled by a sensor on the motherboard which speeds up the fan if it detects the CPU getting too warm. If the heat sink is clogged with dust, the fan may run at a high speed all the time (generating a lot of noise) in an attempt to keep the CPU cool. CPU intensive tasks such as working with audio or video files or playing action games will also cause the CPU to generate more heat and the fan to spin faster. If the heat sink were not attached or not properly installed, the CPU would overheat and shut down within seconds. If the CPU fan stopped running, the CPU would still get some cooling from the heat sink alone, but would eventually overheat and lock the system up.

	Top of a typical CPU. This is an Intel Celeron (Pentium 4 class) CPU running at 2Ghz.
	Bottom of the CPU shown above. The CPU has hundreds of tiny pins that connect to the socket on the motherboard.
	CPU sitting in front of its heat sink. The heat sink is installed on top of the CPU and is clamped down to ensure that the CPU's heat can easily move into the heat sink.
	This is the CPU socket on the motherboard. The brown lever below the beige socket lifts up to allow the CPU's pins to be inserted in the socket. This type of socket is called a ZIF socket, short for "zero insertion force". The CPU's pins are very delicate and would not allow the installer to exert much force on them trying to seat the CPU. With this type of socket, the CPU just drops in with no resistance. The brown lever is then flipped down to lock the CPU in place.
	The CPU after being inserted into the socket
	The heat sink has now been installed on top of the CPU and clamped down