• summaries of what the various directories are "intended" for
  • [authoritative] Filesystem Hierarchy Standard
  • [authoritative] hier(7) — this has existed since V7 in 1979
  • [authoritative] Solaris's filesystem(5) or filesystem(7)
  • [authoritative] XDG's Base Directory Specification (full specification) ("what is XDG?")
  • wikipedia (more)
• home-grown scripts are SUPPOSED to go in /usr/local/, instead of /usr/
  • though some people dispute this
• /tmp/ is supposed to be cleaned out every boot, while files in /var/tmp/ stick around until manually removed [1] [2]