A File system is the organized method used to store and access data on a storage device. This is also loosely referred to as memory.
A File system is an essential element to data storage. It is written on a storage media by writing blocks of information which is then accessed by an index using a block numbering system. The full block will be read into RAM memory and then accessed directly by location. The size of the blocks are generally fixed when the media is initially formatted. There is some overhead associated with the formatting which includes space between the blocks on a disk drive and error detection correction information.
The indexing system used to find the blocks is often called a FAT, or File Allocation Table. This table is stored in a fixed location on the disk so that it can be easily found. It contains pointers to the actual blocks.
 Types of File Systems
At a basic level there are two kinds of file systems, those that support hierarchy (folders/directories) and those that don't. Another way to identify file systems are to name how they initially formatted. There are several types of file systems in use for disk drives and these are also mimicked by SD cards and USB storage devices.
It addition you can identify file systems by how they are used as indicated below:
Portable storage will generally be formatted as FAT12, FAT16, FAT32, or exFAT. Media with a capacity of more than 2 GB must be formatted using FAT32 or exFAT and capacities beyond 32 GB are should be exFAT. In some cases the NTFS files system may also be seen. Other kinds of filesystems are on Unix/Linux systems. A Unix filesystem is characterized by its ability to support links allowing a file to be accessed in multiple locations withing a file hierarchy.
 Camera File System
Digital cameras and even Smartphones use a common file system to permit the exchange of pictures, videos and audio files. It is called the Design rule for Camera File system (DCF) and is a JEITA specification (number CP-3461) which defines a file system for digital cameras, including the directory structure, file naming method, character set, file format, and metadata format. This file system extends the basic storage formats by specifying specific folder names and hierarchy to allow pictures to be found by other devices such as computers, printers, etc. Generally, access to this file system is automatically gained when the unit is plugged into a USB port. This is part of the Exif specification.
See Design rule for Camera File system for specific rules that must be followed.