Important directories could be (RH 7.0 oriented):
/ Root directory
/bin Basic commands
/boot Boot files
/dev Devices (inherit)
/etc Configuration files (inherit)
/etc/X11 X11 configuration files (inherit)
/etc/cron.* Configuration files for scheduled tasks
/etc/init.d Scripts to manage services
/etc/rc* Scripts to manage services in runlevels
/etc/sysconfig System configuration files (inherit)
/etc/httpd WWW server configuration files (inherit)
/home User home directories
/lib Basic library files
/lib/modules Kernel module files (inherit)
/lost+found Crash recovery directory
/mnt Mounted partitions
/opt Optional packages
/proc Kernel interface directory
/root Administrator's directory
/sbin Basic system commands
/tmp Temporary files directory
/usr User utilities directory
/usr/local/ Local system directory
/usr/...
/var ...