/sbin - This directory contains all the binaries that are essential to the working of the system. These include system administration as well as maintenance and hardware configuration programs. Find lilo, fdisk, init,
ifconfig etc here. These are the essential programs that are required by all the users. Another directory that contains system binaries is /usr/sbin. This directory contains other binaries of use to the system administrator. This is where you will find the network daemons for your system along with other binaries that only the system administrator has access to, but which are not required for system maintenance, repair etc.
/bin - In contrast to /sbin, the bin directory contains several useful commands that are used by both the system administrator as well as non-privileged users. This directory usually contains the shells like bash, csh etc. as well as much used commands like cp, mv, rm, cat, ls. There also is /usr/bin, which contains other user binaries. These binaries on the other hand are not essential for the user. The binaries in /bin however, a user cannot do without.
/boot - This directory contains the system.map file as well as the Linux kernel. Lilo places the boot sector backups in this directory.
/dev - This is a very interesting directory that highlights one important characteristic of the Linux filesystem - everything is a file or a directory. Look through this directory and you should see hda1, hda2 etc, which represent the various partitions on the first master drive of the system.
/etc - This directory contains all the configuration files for your system. Under this directory will be X11 sub-directory which contains the configuration files for X. More importantly, the /etc/rc.d directory contains the system startup scripts.
/home - Linux is a multi-user environment so each user is also assigned a specific directory which is accessible only to them and the system administrator. These are the user home directories, which can be found under /home/username.
/lib - This contains all the shared libraries that are required by system programs. Windows equivalent to a shared library would be a DLL file.
/opt - This directory contains all the software and add-on packages that are not part of the default installation.
/usr - This is one of the most important directories in the system as it contains all the user binaries. X and its supporting libraries can be found here. User programs like telnet, ftp etc are also placed here.
/var - This directory contains spooling data like mail and also the output from the printer daemon. The system logs are also kept here in /var/log/messages.