The File Allocation Table (FAT) is a file system structure used by operating systems to manage how files are stored on a disk. It keeps track of which clusters on the disk are used, which are free, and the location of each file's data. This allows the system to efficiently read and write files without losing track of where they are stored. FAT is commonly associated with older systems and is still used in various forms, such as FAT16, FAT32, and exFAT. These variations differ in the maximum size of files and partitions they can handle, making them suitable for different storage devices, including USB drives and memory cards.