/ Root directory of the file system.
/bin/ User utilities fundamental to both single-user and multi-user environments.
/boot/ Programs and configuration files used during operating system bootstrap.
/boot/defaults/ Default bootstrapping configuration files; see loader.conf(5).
/dev/ Device nodes; see intro(4).
/etc/ System configuration files and scripts.
/etc/defaults/ Default system configuration files; see rc(8).
/etc/mail/ Configuration files for mail transport agents such as sendmail(8).
/etc/namedb/ named configuration files; see named(8).
/etc/periodic/ Scripts that are run daily, weekly, and monthly, via cron(8); see periodic(8).
/etc/ppp/ ppp configuration files; see ppp(8).
/mnt/ Empty directory commonly used by system administrators as a temporary mount point.
/proc/ Process file system; see procfs(5), mount_procfs(8).
/root/ Home directory for the root account.
/sbin/ System programs and administration utilities fundamental to both single-user and multi-user environments.
/stand/ Programs used in a standalone environment.
/tmp/ Temporary files, usually a mfs(8) memory-based file system (the contents of /tmp are usually NOT preserved across a system reboot).
/usr/ The majority of user utilities and applications.
/usr/bin/ Common utilities, programming tools, and applications.
/usr/include/ Standard C include files.
/usr/lib/ Archive libraries.
/usr/libdata/ Miscellaneous utility data files.
/usr/libexec/ System daemons & system utilities (executed by other programs).
/usr/local/ Local executables, libraries, etc. Also used as the default destination for the FreeBSD ports framework. Within /usr/local, the general layout sketched out by hier(7) for /usr should be used. Exceptions are the man directory, which is directly under /usr/local rather than under /usr/local/share, and the ports documentation is in share/doc/port.
/usr/obj/ Architecture-specific target tree produced by building the /usr/src tree.
/usr/ports The FreeBSD ports collection (optional).
/usr/sbin/ System daemons & system utilities (executed by users).
/usr/share/ Architecture-independent files.
/usr/src/ BSD and/or local source files.
/usr/X11R6/ X11R6 distribution executables, libraries, etc (optional).
/var/ Multi-purpose log, temporary, transient, and spool files.
/var/log/ Miscellaneous system log files.
/var/mail/ User mailbox files.
/var/spool/ Miscellaneous printer and mail system spooling directories.
/var/tmp/ Temporary files that are kept between system reboots.
/var/yp NIS maps.