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
macOS How To
ls, apropos(1), whatis, whereis(1), finger(1), which, find, grep, fsck