The power supply is what powers everything in your system. It's main purpose is to convert the electricity from your wall outlet from A/C to D/C. This is done using a rectifying circuit, which creates heat. This is why most power supplies have a fan. They come in different shapes and sizes and have a multitude of different connectors. Some are modular, which means you can connect only the cables you need for a cleaner look. Choose one that provides enough power for everything in your system (power is rated in watts) and make sure it fits your case and has all the right connectors for your components (motherboard, fans, hard drive, etc.) Once again, we're here to help you so contact us if you have questions!