What Exactly Is a File System?

Definition of file system, what they're for, and common ones used today

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Updated on August 28, 2023

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

A file system can be thought of as an index or database containing the physical location of every piece of data on the 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 employs some type of file system. This includes your Windows computer, your Mac, your smartphone, your bank's ATM—even the computer in your car!

Picture of the inside of a Seagate hard drive — Getty Images / Csaba Bajko Photography

Windows File Systems

The Microsoft Windows operating systems have always supported various versions of the FAT file system. FAT stands for File Allocation Table, a term that describes what it does: maintains a table of each file's space allocation.

In addition to FAT, all Windows operating systems since Windows NT support a newer file system called NTFS—New Technology File System. For Windows NT, the NT stood for new technology.

All modern versions of Windows also support exFAT, which is designed for flash drives.

ReFS (Resilient File System) is a newer file system for Windows 11, 10, and 8 that includes features not available with NTFS, but it's currently limited in several ways. You can see which versions of Windows support each version of ReFS in this table.

A file system is set up 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 sectors. Sectors marked as unused can store data, typically 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.

Over time, because of 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).

What's meant 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't bear 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 kinds of file systems dot the operating-system horizon, 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 the drive is split between NTFS and FAT, but that there are two separate partitions that use the same physical disk.

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.

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