A boot loader is a small program that runs when a computer is powered on. Its main job is to load the operating system into memory so that the computer can start functioning. The boot loader is typically stored in the computer's firmware or on a storage device, and it initializes the hardware components before handing control over to the operating system. There are different types of boot loaders, such as the GRUB for Linux systems and the Windows Boot Manager for Windows systems. Each boot loader has its own set of features, allowing users to select different operating systems or configurations during the startup process.