What is a File System?

Definition of File System, What They're For, and Common Ones Used Today

Picture of the inside of a Seagate hard drive
© Csaba Bajko photography. Szentendre / Hungary / Moment / Getty Images

Computers use particular kinds of file systems (sometimes abbreviated FS) to store and organize data on media, such as a hard drive, the CDs, DVDs, and BDs in an optical drive or on a flash drive.

A file system can be thought of as an index or database containing the physical location of every piece of data on the hard drive or another storage device. The data is usually organized in folders called directories, which can contain other folders and files.

Any place that a computer or other electronic device stores data is employing the use of some type of file system. This includes your Windows computer, your Mac, your smartphone, your bank's ATM... even the computer in your car!

Windows File Systems

The Microsoft Windows operating systems have always supported, and still do support, various versions of the FAT (File Allocation Table) file system.

In addition to FAT, all Microsoft Windows operating systems since Windows NT support a newer file system called NTFS (New Technology File System).

All modern versions of Windows also support exFAT, a file system designed for flash drives.

A file system is a setup on a drive during a format. See How To Format a Hard Drive for more information.

More About File Systems

Files on a storage device are kept in what's called sectors. Sectors marked as unused can be utilized to store data, which is typically done in groups of sectors called blocks.

It's the file system that identifies the size and position of the files as well as which sectors are ready to be used.

Tip: Over time, due to the way the file system stores data, writing to and deleting from a storage device causes fragmentation because of the gaps that inevitably occur between different parts of a file.

A free defrag utility can help fix that.

Without a structure for organizing files, it not only would be next to impossible to remove installed programs and retrieve specific files, but no two files could exist with the same name because everything might be in the same folder (which is one reason folders are so useful).

Note: What I mean by files with the same name is like an image, for example. The file IMG123.jpg can exist in hundreds of folders because each folder is used to separate the file, so there isn't a conflict. However, files can not contain the same name if they're in the same directory.

A file system doesn't just store the files but also information about them, like the sector block size, fragment information, file size, attributes, file name, file location, and directory hierarchy.

Some operating systems other than Windows also take advantage of FAT and NTFS but many different kinds of file systems exist, like HFS+ used in Apple product like iOS and macOS. Wikipedia has a comprehensive list of file systems if you're more interested in the topic.

Sometimes, the term "file system" is used in the context of partitions. For example, saying "there are two file systems on my hard drive" doesn't mean that the drive is split between NTFS and FAT, but that there are two separate partitions that are using the file system.

Most applications you come into contact with require a file system in order to work, so every partition should have one. Also, programs are file system-dependant, meaning you can't use a program on Windows if it was built for use in macOS.