The filesystem hierarchy standard (FHS) is a filesystem standard that is used in most Unix-like OS, most notably Linux.
It is maintained by the Linux Foundation.
Examples of Unix-like OSs not using FHS intentionally are GoboLinux and NixOS.
Booting
/boot contains the files for boot configuration, cores and other necessary files to boot the device.
Program Data
/bin contains the built-in binary executables.
/etc/services is a file that contains the services that are run in a given moment.
/etc/inetd is a file that contains the services that are run during initialization.
/proc contains information about the processes that are currently running
/tmp contains the boot program temporary data and the most volatile temporary data.
/var also contains the programs’ temporary data, but it may be stored longer and not during booting.
Device Data
/dev includes all the devices.
/mnt includes only removable media.
Users
/etc/passwd contains users.
The character * is used for users that have interactive login disabled.
The use of shadowed password files more from hashes from file /etc/passwd to the most restricted file /etc/shadow.
When shadowed passwords are used, passwd file is left with a simple ‘x’ as content.