I didn't follow rev's link, but I'm sure it is valid. Linux (and all unicies) are unique in the way almost every device is viewed as a file. For a quick run down:
/dev contains devices. Block devices (harddrives et.al) need to be mounted. They are normally mounted under /mnt. /dev also contains all other devices. Soundcard: /dev/pcm, ethernet /dev/ethX, consoles: /dev/ptyX, random numbers: /dev/urandom. At least one partition is mounted under /. You normally have another partition mounted under /boot. type "mount" as root to see how your partitions are setup.
/proc contains files that store information about your system or devices. Try "cat /dev/cpuinfo" or "cat /dev/partitions" to get the idea. All the numbered files and directories are processes. /var contains various things like logfiles, www directory, and some data files shared by programs.
/bin contains binaries accessable to everyone. /sbin contains binaries for the super-user. /usr contains files used mainly by local users. This includes /usr/bin and /usr/share for files shared by programs and users.
/lib contains libraries. These can be thought of as dlls in the windows world. /root is the super-user's home directory. /tmp contains temporary files. That is, /tmp can be considered volatile.
Hope that helps. Play around with the file system using tools such as cat, du, etc. It's also fun to string some things together. For example:
cat /dev/urandom > /dev/dsp
BTW, I don't know why the system won't let me mount schoolgirls.