An PSU, or Power Supply Unit, is a hardware component that supplies power to an electronic device such as a computer. PSUs are typically used to convert AC, or Alternating Current, from the electric company to DC, or Direct Current, which is the type of electricity used in most electronic devices.
PSUs are typically categorized into two types: external and internal. Internal PSUs are those found in desktop computers, monitors, TVs, etc where the power supply is integrated into the electronic device and only a power cord is needed to connect to the wall socket.
External PSUs are those found in laptops or other devices where the PSU is sitting somewhere between the electronic component and the wall outlet. Oftentimes, these are also referred to as power adapters or power bricks.