In DOS systems, FATs are stored just after the boot sector.

The FAT system for older versions of Windows 95 is called FAT16, and the one for new versions of Windows 95 and Windows 98 is called FAT32.

Hard Disk Logical Structures and File Systems 
Describes "standard" file systems used on PCs, with particular focus on the FAT file system and its variants. This page is from "The PC Guide."

FAT File System Disk Volume Structures
Describes the disk volume structures used in the FAT file system. This page is from "The PC Guide."

File Systems (FAT, HPFS, NTFS)
Contains brief technical descriptions of the FAT file system, VFAT, HPFS, and NTFS.

FAT types and cluster sizes
This Microsoft Knowledge Base article discusses FAT types and cluster size and their relation to logical drive size, including information on double-spaced drives.

Understanding File System Options
Whether you're looking for a refresher on files systems or want to learn the difference between FAT file systems vs. NTFS file systems, this tutorial explains the ins and outs, and pros and cons of each.