Short for File Allocation Table, FAT is a method used by Microsoft operating systems to keep track of the contents of a disk; the table is a chart of numbers that correspond to cluster addresses on the hard drive.
FAT12: The oldest type of File Allocation Table that uses 12-bit binary system. A hard disk drive formatted using FAT12 can use a maximum of approximately 16,736,256 volume size and today is no longer used. If your computer is running Windows 95 or above and your FAT within FDISK is being displayed as FAT12 it is likely that your hard disk drive is corrupted, bad or has a computer virus.
FAT16: FAT utilizing a 16-bit binary system. Commonly found with Windows 3.x through Windows 95. FAT16, while becoming obsolete by FAT32, is still commonly found.
FAT32: Enhanced File Allocation Table utilizing a 28-bit binary system, first used in Windows 95 OSR2 and more so in Windows 98, that saves disk space by using 4k Cluster. See FAT32 Page for extended information about FAT32.