/ the root directory. The top level directory of everything.
/bin essential CLI binary executables. On RHEL7, it is a symlink to /usr/bin/
/boot contains startup files, including the kernel.
/dev hardware and software device drivers
/etc/ basic config files
/home/ home directories for most users
/lib program libraries for the kernel and other utilities. On RHEL7, it is a symlink to /usr/lib
/lib64/ Like lib, but 64 bit versions. On RHEL7, symlink to /usr/lib64.
/media/ mount point for removable media like DVDs.
/misc/ standard mount point for directories mounted by the automounter.
/mnt/ mount point for temporary mounted filesystems
/net/ standard mount point for network shares mounted by the automounter
/opt/ common location for third party application files
/proc/ Virtual filesystem for current kernel processes, IRQ ports, etc.
/root/ home directory of the root account
/run/ a TMPFS filesystem for files that should not persist after a reboot. On RHEl7, this replaces /var/run/ and /var/run is now a symlink to /run/
/sbin/ contains system administration binary executables. On RHEL7, this is a symlink to /usr/bin/
/smb/ standard mount point for SMB shares mounted by the automounter
/srv/ commonly used by network servers on non-Red Hat distros
/sys/ is similar to /proc/ and exposes info about devices, drivers, etc.
/tmp/ contains temporary files. By default RHEL purges this directory periodically
/usr/ contains programs and read-only data.
/var/ contains variable data, including logfiles, databases, printer spools, etc.