Files, Folders and File Systems Explained

A storage drive is, underneath, an enormous row of numbered spaces that each hold a few bytes. Nobody wants to remember that "my holiday photos start at space 48,201,377". People want names, and they want to group related things together. The software that makes this possible is the file system.

This entry explains the three ideas that organize almost everything you store: files, folders and the file system that connects them. It also covers paths and extensions, which you meet every time you save or search for something.

What a file is

A file is a named collection of data stored for the long term. A letter, a spreadsheet, a photo, a song and a program are all files. To the computer, each one is just a sequence of bytes with a name, a size, and some extra facts such as when it was last changed.

Two parts of a file are worth separating:

• The content. The bytes themselves, such as the pixels of a picture.
• The metadata. Information about the file: its name, size, dates, and who may open it. Metadata is stored by the file system, usually apart from the content.

File extensions

Many file names end with a dot and a few letters, such as .txt, .jpg or .pdf. This ending is the extension. It hints at the kind of data inside, so the system knows which program should open the file. Renaming a file to a new extension does not convert it. Changing notes.txt to notes.jpg just gives the same text a misleading name, and a picture viewer will fail to open it.

Some systems hide extensions by default. If two files look alike and you want to tell them apart, turn on the option to show extensions in your file manager.

What a folder is

A folder, also called a directory, is a container that holds files and other folders. Folders can sit inside folders, which creates a tree. At the top is a single starting point called the root.

Take a person planning a trip to Lisbon. They might organize things like this:

Trips
  Lisbon
    tickets.pdf
    hotel-booking.pdf
    packing-list.txt
    Photos
      tram.jpg
      harbor.jpg
  Oslo
    itinerary.txt

Photos is a folder inside Lisbon, which is inside Trips. Grouping files this way is not just tidiness. It also lets you move, copy or back up a whole trip in a single action.

Paths: the full address of a file

A path is the route from the top of the tree to a file. It lists each folder on the way, separated by a slash character. The path of the tram photo above could be written like this on two common systems:

Windows uses a backslash and starts with a drive letter such as C:. macOS and Linux use a forward slash and start from a single root written as /. Both are the same idea in a different spelling.

Absolute and relative paths

An absolute path starts from the root, so it always points to the same place. A relative path starts from wherever you are now. If you are already inside Lisbon, then Photos/tram.jpg is enough. In many systems, two dots (..) mean "the folder above this one". Relative paths are shorter, and you will meet them often when you use the command line.

What a file system does

The file system is the part of the operating system that organizes storage into files and folders. Without it, a drive would be a pile of unlabeled bytes. Its main tasks are:

• Tracking where each file lives. Big files are often split into pieces stored in different places. The file system keeps the map that joins them again.
• Keeping names and the folder tree. It records which file belongs in which folder.
• Storing metadata, such as size, dates and permissions.
• Managing free space, so that new files go into unused areas and deleted files free their space for reuse.
• Enforcing permissions, so that one user cannot open another user's private files unless allowed.

The operating system you use asks the file system to do these things whenever you save or open something. If you want the bigger picture, see what an operating system does and how a computer works.

Different file systems for different needs

There is more than one file system, and each has strengths. Common examples include NTFS on Windows, APFS on Apple devices, ext4 on many Linux systems, and exFAT, which is popular on memory cards and external drives because several systems can read it. A drive formatted for one system may not be readable by another without extra help, which is why exFAT is so often chosen for drives that move between computers. Some older formats also limit the size of a single file, so a very large video may fail to copy onto them.

You rarely need to pick a file system for everyday work. It matters when you set up a new drive, share one between systems, or work in IT.

What formatting and deleting really mean

Formatting a drive means creating a fresh file system on it, so that the drive has an empty folder tree ready for use. A quick format usually resets the map and leaves much of the old data in place until it is overwritten. A full format does more work. In both cases, formatting is a serious step, and you should do it only on a drive whose contents you have backed up or no longer need.

Deleting a file works in a similar way. Most systems first move it to a recycle bin or trash folder, which is easy to undo. Emptying the bin tells the file system the space is free again. The bytes may stay on the drive until something new is written over them. This is why deleted files can sometimes be recovered, and also why securely erasing a drive needs special tools.

Common mistakes

• Saving everything on the desktop. It works for a week and then becomes a mess. A simple folder tree is easier to search and to back up.
• Changing an extension to convert a file. This only changes the label. Use a program that actually converts the content.
• Using very long or unclear names. Names like final-final2 cause confusion. Add a date or a version, and keep names short and consistent.
• Mixing up deleting with erasing. Putting a file in the bin does not remove it, and even emptying the bin may leave the bytes on the drive for a while.
• Formatting the wrong drive. Check the drive's name and size before you confirm. A format erases access to what was there.
• Having one copy of important files. A drive can fail. Keep another copy elsewhere.

FAQ

What is the difference between a file and a folder?

A file holds data, such as a document. A folder holds files and other folders and has no content of its own. It only organizes.

Why can't I open a file from another computer?

Common reasons are that no installed program understands the file type, the drive uses a file system your system cannot read, or you do not have permission to open it.

Does a bigger drive mean a faster computer?

Not by itself. Capacity is how much you can store. Speed depends on the type of drive and the rest of the computer.

What is the root folder?

It is the top of the tree, the folder that contains all the others. On Windows each drive has its own root, such as C:\. On macOS and Linux there is one root, written /.

Are folders and directories the same thing?

Yes. "Folder" is the friendlier word used in graphical interfaces. "Directory" is the older word, and you will see it in commands and documentation. More topics like this are in the Computing and Networks area.