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.