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The Free On-line Dictionary of Computing (foldoc)

User. The "/usr" directory hierarchy on {Unix} systems. Once
upon a time, in the early days of Unix, this area actually
held users' home directories and files. Since these tend to
expand much faster than system files, /usr would be mounted on
the biggest disk on the system. The root directory, "/" in
contrast, contains only what is needed to {boot} the {kernel},
after which /usr and other disks could be mounted as part of
the multi-user start-up process.

/usr has been used as the "everything else" area, with many
"system" files such as compiler libraries (/usr/include,
/usr/lib), utilty programs (/usr/bin, /usr/ucb), games
(/usr/games), local additions (/usr/local), manuals
(/usr/man), temporary files and queues for various {daemons}
(/usr/spool). These optional extras have grown in size as
Unix has evolved and disks have dropped in price. Under later
versions of {SunOS}, the user files have fled /usr altogether
for a new "/home" {partition} and temporary files have moved
to "/var". This allows /usr to be mounted read-only with some
gain in security and performance since access times are not
updated for files on read-only file systems.