The Filesystem Hierarchy Standard entry in Wikipedia helped me answer the same question when I had it, plus it has a very explanatory table.

The Filesystem Hierarchy Standard entry in Wikipedia helped me answer the same question when I had it, plus it has a very explanatory table.

Excerpt from that page¹:

/bin        Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.
/usr/bin    Non-essential command binaries (not needed in single user mode); for all users.
/usr/local  Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin, lib, share
/usr/sbin   Non-essential system binaries, e.g., daemons for various network-services.
/sbin       Essential system binaries, e.g., fsck, init, route.

¹ _{Retrieved on June 19, 2019; permalink.}