The Filesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout of a UNIX system. It has been made popular by its use in Linux distributions, but it is used by other UNIX variants as well. The Linux Standard Base (LSB) refers to it as a standard.
|/usr||Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications|
|/local||Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin/, lib/, share/. /usr/local/ is for data that must be stored on the local host. Most of the time /usr/local/ is used for installing software/data that are not part of the standard operating system distribution.|