Filesystem architecture

/bin
Essential commands to boot the system or in single-user mode
Essential commands required by all system users

/sbin
Essential binaries related to system administration

On some new distributions /usr/bin and /bin are symbolically linked together, as are /usr/sbin and /sbin.

/proc

Runtime system information. Not real but virtual files which dynamically reflect the kernel structure and configuration information.

/dev
Device nodes, except for network devices.
Note: /dev/null is a This pseudofile also called the "bit bucket" or "black hole". Redirecting the output there will lose it, therefore it is a fake writing. But the error will be displayed on the screen, unless the redirection is including stnderr:
$ commnad >& /dev/null

/var
Variable files which change in dimension while the system is running. For this reason it's often put in a dedicated partition so its variability can't compromise the free space necessary to the main system. It can contain network services directories.

/etc
Configuration files, no binary files.

/boot
Files needed to boot the system. For every alternative kernel installed on the system there are four files:

• vmlinuz
  The compressed Linux kernel, required for booting.
• initramfs
  The initial ram filesystem, required for booting, sometimes called initrd, not initramfs.
• config
  The kernel configuration file, only used for debugging and bookkeeping.
• System.map
  Kernel symbol table, only used for debugging.
• GEUB files
  Such as /boot/grub/grub.conf or /boot/grub2/grub2.cfg

Each of these files has a kernel version appended to its name.

/lib
32 bit libraries.

/lib64
64 bit libraries.

Some of them are dynamically loaded libraries  (DL) also known as Shared Libraries or Shared Objects (SO), which are loaded after the startup of a program. They are requested by plugins or modules. Kernel modules are located in /lib/modules/<kernel-version-number>.

/mnt
Ever more replaced by /run.

/opt
Optional software packages

/sys

Virtual pseudo-filesystem for information about the system and the hardware. It can be used to alter system parameters and debug.

/sys
Site-specific data served by the system. It gives users the location of data files for a particular service, such as FTP, WWW, or CVS. Data that only relate to a specific user should go in the /home/ directory.

/usr
Multi-user programs and data.

• /usr/include - Header files used to compile applications
• /usr/lib - Libraries for programs in /usr/bin and /usr/sbin
• /usr/lib64 - 64-bit libraries for 64-bit programs in /usr/bin and /usr/sbin
• /usr/sbin - Non-essential system binaries, such as system daemons
• /usr/share - Shared data used by applications, generally architecture-independent
• /usr/src - Source code, usually for the Linux kernel
• /usr/local - Data and programs specific to the local machine; subdirectories include bin, sbin, lib, share, include, etc.
• /usr/bin - This is the primary directory of executable commands on the system