A bootable device is any hardware that can load an operating system when a computer starts up. Common examples include USB flash drives, CDs, DVDs, and external hard drives. These devices contain the necessary files to initiate the boot process, allowing the computer to run software and perform tasks. When a computer is powered on, it checks for bootable devices in a specific order, known as the boot sequence. If a bootable device is detected, the system will load the operating system from it. This process is essential for installing new operating systems or troubleshooting existing ones.