FAT

FAT - File Allocation Table. This is the underlying filesystem of DOS and Windows. It comes in 3 varieties. 12-bit (FAT-12), 16-bit (FAT-16) and 32-bit (FAT-32). The FAT filesystem is comprised of a File Allocation Table (F.A.T.), a backup F.A.T.. The FAT defines files and folders. Each file is made up of one or many file allocation units called clusters. Each cluster is a fixed size. In FAT-32 the size is usually 4096 bytes, but it can be anywhere from 512 bytes to 64 kbytes. The clusters specify the smallest amount of space that can be allocated to a file.

Okay, it's a bit hard for me to explain, but I tried my best. Anyone want to give it a shot?

In other words, the FAT system is the table of contents of the files on a hard drive. The hard drive stores information on random parts of the disc inside of it. The only way to know where something's located is the check the FAT system. It tells the computer to look for a marker and then from there to the location of the file.