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?