A USB controller is a hardware component that manages the communication between a computer and various USB devices, such as keyboards, mice, and external drives. It allows these devices to connect and transfer data efficiently, enabling users to expand their computer's functionality. These controllers can be built into the computer's motherboard or exist as separate expansion cards. They support different versions of USB, such as USB 2.0, USB 3.0, and USB 3.1, each offering varying speeds and capabilities. This ensures compatibility with a wide range of devices, enhancing user experience and connectivity options.