Overview
Most Linux distributions follow the FHS and declare it their own policy to maintain FHS compliance.
When the FHS was created, other UNIX and Unix-like operating systems already had their own standards, notably the hier(7) description of file system layout that has existed since the release of Version 7 Unix (in 1979), or the SunOS filesystem(7), later Solaris filesystem(5).
Modern Linux distributions include a /sys directory as a virtual filesystem (sysfs, comparable to /proc, which is a procfs), which stores and allows modification of the devices connected to the system, whereas many traditional UNIX and Unix-like operating systems use /sys as a symbolic link to the kernel source tree.
Some Linux systems such as GoboLinux and Syllable Server use a completely different approach from the FHS.
Read more about this topic: Filesystem Hierarchy Standard