On Linux, everything on the system is represented with a file---Keyboards, disk drives, robotic arms, running programs and the rest: All files. Naturally, a Linux system needs a lot of files---And a sensible way to organize them.

The File Hierarchy Standard (FHS for short) is the generally agreed upon way of organizing the files of a Linux system. I say "generally" because some systems do deviate here and there. Thus follows an outline of the FHS with some notes about those little idiosyncrasies in the form of a bulleted list. Each item represents a directory containing the items tabulated ahead of it.

• / "Root" Everything on the system.
  • /bin Essential binaries (Sometimes a link to /usr/bin).
  • /boot Boot loader files.
  • /dev Device files.
  • /etc System-wide configuration files.
  • /home Non-root user directories.
    • /home/your-user-name Your user directory.
  • /lib Essential libraries (Sometimes a link to /usr/lib).
  • /media Mount points for removable media (Alternatively, /run/media).
  • /mnt Temporarily mounted file systems.
  • /opt Optional application files (Standalone installers may use this).
  • /proc Process and kernel information files.
  • /root Home directory of the root user.
  • /run Run-time data since last boot.
  • /sbin Essential system binaries (Sometimes a link to /bin).
  • /srv Data served (vis á vis HTTP, FTP, etc.) by the system.
  • /sys Information about devices, drivers, and kernel features.
  • /tmp Temporary files. Your browser may cache web pages here.
  • /usr Read only user data: Applications and their libraries, mostly.
    • /usr/bin User application binaries.
    • /usr/lib User application libraries.
    • /usr/include Development headers.
    • /usr/share Non-instruction user application files.
    • /usr/local Third-party user applications data.
  • /var Files expected to change frequently, caches, logs, spools, etc.

In your Linux journey, you may encounter all manner of unusual file system choices. GoboLinux, for instance, makes a distinct hierarchy for each program! And NixOS completely eschews the FHS in favor of modularity. We won't worry about all that here though. For the most part, Linux operating systems stick to the FHS. Where they do not, the variations are either slight or rigorously documented.