A historical sketch of the filesystem hierarchy. (The modern macOS filesystem adds many other folders to this basic structure).
/ root directory of the filesystem /bin/ user utilities fundamental to both single-user and multi-user environments /dev/ block and character device files fd/ file descriptor files; see fd(4) /etc/ system configuration files and scripts /mach_kernel kernel executable (the operating system loaded into memory at boot time). /sbin/ system programs and administration utilities fundamental to both single- user and multi-user environments /tmp/ temporary files /usr/ contains the majority of user utilities and applications bin/ common utilities, programming tools, and applications include/ standard C include files arpa/ C include files for Internet service protocols hfs/ C include files for HFS machine/ machine specific C include files net/ misc network C include files netinet/ C include files for Internet standard protocols; see inet(4) nfs/ C include files for NFS (Network File System) objc/ C include files for Objective-C protocols/ C include files for Berkeley service protocols sys/ system C include files (kernel data structures) ufs/ C include files for UFS lib/ archive libraries libexec/ system daemons & system utilities (executed by other programs) local/ executables, libraries, etc. not included by the basic operating system sbin/ system daemons & system utilities (executed by users) share/ architecture-independent data files calendar/ a variety of pre-fab calendar files; see calendar dict/ word lists; see look(1) web2 words from Webster's 2nd International words common words man/ manual pages misc/ misc system-wide ascii text files mk/ templates for make; see make(1) skel/ example . (dot) files for new accounts tabset/ tab description files for a variety of terminals; used in the termcap file; see termcap(5) zoneinfo/ timezone configuration information; see tzfile(5) /var/ multi-purpose log, temporary, transient, and spool files at/ timed command scheduling files; see at(1) backups/ misc. backup files db/ misc. automatically generated system-specific database files log/ misc. system log files mail/ user mailbox files run/ system information files describing various info about system since it was booted utmpx database of current users; see utmpx(5) rwho/ rwho data files; see rwhod(8), rwho(1), and ruptime(1) spool/ misc. printer and mail system spooling directories mqueue/ undelivered mail queue; see sendmail(8) tmp/ temporary files that are kept between system reboots folders/ per-user temporary files and caches
ls, apropos(1), whatis, whereis(1), finger(1), which, find, grep, fsck