Arch Linux directory structure (filesystem hierarchy)

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux operating systems. Most Linux distributions generally follow the FHS, including Arch, with some modifications.

Directory Description
/ The root filesystem, represented by the slash symbol by itself (/), is the primary filesystem from which all other filesystems stem; the top of the hierarchy. All files and directories appear under the root directory "/", even if they are stored on different physical devices. The contents of the root filesystem must be adequate to boot, restore, recover, and/or repair the system.
Unsharable, static directory containing the kernel and ramdisk images as well as the bootloader configuration file, and bootloader stages. /boot also stores data that is used before the kernel begins executing userspace programs. This may include saved master boot sectors and sector map files.
Essential device nodes created by udev during the boot process and as machine hardware is discovered by events. This directory highlights one important aspect of the UNIX filesystem - everything is a file or a directory. Exploring this directory will reveal many files, each representing a hardware component of the system. The majority of devices are either block or character devices; however other types of devices exist and can be created. In general, 'block devices' are devices that store or hold data, whereas 'character devices' can be thought of as devices that transmit or transfer data. For example, hard disk drives and optical drives are categorized as block devices while serial ports, mice and USB ports are all character devices.
Host-specific, unsharable configuration files shall be placed in the /etc directory. If more than one configuration file is required for an application, it is customary to use a subdirectory in order to keep the /etc/ area as clean as possible. It is considered good practice to make frequent backups of this directory as it contains all system-related configuration files.

UNIX is a multi-user environment. Therefore, each user is also assigned a specific directory that is accessible only to them and to the root user. These are the user home directories, which can be found under '/home/$USER' (~/). Within their home directory, a user can write files, delete them, install programs, etc. Users' home directories contain their data and personal configuration files, the so-called 'dot files' (their name is preceded by a dot), which are 'hidden'. To view dotfiles, enable the appropriate option in your file manager or run ls with the -a switch. If there is a conflict between personal and system wide configuration files, the settings in the personal file will prevail. Dotfiles most likely to be altered by the end user include .xinitrc and .bashrc files. The configuration files for xinit and Bash respectively. They allow the user the ability to change the window manager to be started upon login and also aliases, user-specified commands and environment variables respectively. When a user is created, their dotfiles shall be taken from the /etc/skel directory where system sample files reside. Directory /home can become quite large as it is typically used for storing downloads, compiling, installing and running programs, mail, collections of multimedia files, etc.
UNIX-like operating systems must execute a proper shutdown sequence. At times, a system might crash or a power failure might take the machine down. Either way, at the next boot, a filesystem check using the fsck program shall be performed. Fsck will go through the system and try to recover any corrupt files that it finds. The result of this recovery operation will be placed in this directory. The files recovered are not likely to be complete or make much sense but there always is a chance that something worthwhile is recovered.
This is a generic mount point for temporary filesystems or devices. Mounting is the process of making a filesystem available to the system. After mounting, files will be accessible under the mount-point. Additional mount-points (subdirectories) may be created under /mnt/. There is no limitation to creating a mount-point anywhere on the system, but by convention and for practicality, littering a file system with mount-points should be avoided.
Packages and large static files that do not fit cleanly into the above GNU filesystem layout can be placed in /opt. A package placing files in the /opt/ directory creates a directory bearing the same name as the package. This directory in turn holds files that otherwise would be scattered throughout the file system. For example, the acrobat package has Browser, Reader, and Resource directories sitting at the same level as the bin directory. This doesn't fit into a normal GNU filesystem layout, so Arch places all the files in a subdirectory of /opt.
Directory /proc is very special in that it is also a virtual filesystem. It is sometimes referred to as the process information pseudo-file system. It doesn't contain 'real' files, but rather, runtime system information (e.g. system memory, devices mounted, hardware configuration, etc). For this reason it can be regarded as a control and information center for the kernel. In fact, quite a lot of system utilities are simply calls to files in this directory. For example, 'lsmod' is the same as 'cat /proc/modules' while 'lspci' is a synonym for 'cat /proc/pci'. By altering files located in this directory, kernel parameters may be read/changed (sysctl) while the system is running. The most distinctive facet about files in this directory is the fact that all of them have a file size of 0, with the exception of kcore, mounts and self.
Home directory of the System Administrator, 'root'. This may be somewhat confusing, ('/root under root') but historically, '/' was root's home directory (hence the name of the Administrator account). To keep things tidier, 'root' eventually got his own home directory. Why not in '/home'? Because '/home' is often located on a different partition or even on another system and would thus be inaccessible to 'root' when - for some reason - only '/' is mounted.
Site-specific data which is served by the system. The main purpose of specifying this is so that users may find the location of the data files for a particular service, and so that services which require a single tree for read-only data, writable data and scripts (such as CGI scripts) can be reasonably placed. Data of interest to a specific user shall be placed in that user's home directory.
This directory contains files that are required temporarily. Many programs use this to create lock files and for temporary storage of data. Do not remove files from this directory unless you know exactly what you are doing! Many of these files are important for currently running programs and deleting them may result in a system crash. On most systems, old files in this directory are cleared out at boot or at daily intervals.
While root is the primary filesystem, /usr is the secondary hierarchy, for user data, containing the majority of (multi-)user utilities and applications. /usr is shareable, read-only data. This means that /usr shall be shareable between various hosts and must not be written to, except by the package manager (installation, update, upgrade). Any information that is host-specific or varies with time is stored elsewhere. Aside from /home/, /usr/ usually contains by far the largest share of data on a system. Hence, this is one of the most important directories in the system as it contains all the user binaries, their documentation, libraries, header files, etc. X and its supporting libraries can be found here. User programs like telnet, ftp, etc., are also placed here. In the original UNIX implementations, /usr/ (for user), was where the home directories of the system's users were placed (that is to say, /usr/someone was then the directory now known as /home/someone). Over time, /usr/ has become where userspace programs and data (as opposed to 'kernelspace' programs and data) reside. The name has not changed, but its meaning has narrowed and lengthened from everything user-related to user-usable programs and data. As such, the backronym 'User System Resources' was created.
Command binaries. This directory contains the vast majority of binaries (applications) on your system. Executables in this directory vary widely. For instance vi, gcc, and gnome-session reside here. Traditionally, this contained only binaries that did not require root privileges and that were not necessary in single-user mode. However, this is no longer enforced and the Arch devs plan to move all binaries here.
Header files needed for compiling userspace source code.
Contains application private data (kernel modules, systemd services, udev rules, etc) and shared library images (the C programming code library). Libraries are collections of frequently used program routines and are readily identifiable through their filename extension of *.so. They are essential for basic system functionality. Kernel modules (drivers) are in the subdirectory /lib/modules/<kernel-version>.
Optional tertiary hierarchy for local data. The original idea behind '/usr/local' was to have a separate ('local') '/usr/' directory on every machine besides '/usr/', which might be mounted read-only from somewhere else. It copies the structure of '/usr/'. These days, '/usr/local/' is widely regarded as a good place in which to keep self-compiled or third-party programs. This directory is empty by default in Arch Linux. It may be used for manually compiled software installations if desired. pacman installs to /usr/, therefore, manually compiled/installed software installed to /usr/local/ may peacefully co-exist with pacman-tracked system software.
This directory contains 'shareable', architecture-independent files (docs, icons, fonts etc). Note, however, that '/usr/share' is generally not intended to be shared by different operating systems or by different releases of the same operating system. Any program or package which contains or requires data that do not need to be modified should store these data in '/usr/share/' (or '/usr/local/share/', if manually installed - see below). It is recommended that a subdirectory be used in /usr/share for this purpose.
The 'linux' sub-directory holds the Linux kernel sources, and header-files.
Variable files, such as logs, spool files, and temporary e-mail files. On Arch, the ABS tree and pacman cache also reside here. Why not put the variable and transient data into /usr/? Because there might be circumstances when /usr/ is mounted as read-only, e.g. if it is on a CD or on another computer. '/var/' contains variable data, i.e. files and directories the system must be able to write to during operation, whereas /usr/ shall only contain static data. Some directories can be put onto separate partitions or systems, e.g. for easier backups, due to network topology or security concerns. Other directories have to be on the root partition, because they are vital for the boot process. 'Mountable' directories are: '/home', '/mnt', '/tmp', '/usr' and '/var'. Essential for booting are: '/bin', '/boot', '/dev', '/etc', '/lib', '/proc' and '/sbin'.
The ABS tree. A ports-like package build system hierarchy containing build scripts within subdirectories corresponding to all installable Arch software.
Application cache data. Such data are locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. The cached files can be deleted without loss of data.
The pacman package cache.
Persistent data modified by programs as they run (e.g. databases, packaging system metadata, etc.).
Log files.
Shareable directory for users' mailboxes.
Information about the running system since last boot, e.g., currently logged-in users and running daemons.
Spool for tasks waiting to be processed (e.g. print queues and unread mail).
Temporary files to be preserved between reboots.