Linux

/ is where everything goes. It is like C:

/home is user files. It is like C:\Documents and Settings

/tmp is temporary files. This is sometimes mounted as a virtual filesystem in RAM. It is like C:\WINDOWS\Temp

/var is files that change a lot. This is sometimes mounted as a virtual filesystem in RAM. There isn't really a Windows equivalent.

/usr is for non-essential programs and libraries. It is like C:\Program Files

/boot is Linux itself, as well as the bootloader.

/bin is essential programs

/lib is essential libraries

/dev gives files for most of the devices plugged into your computer. It is like the special Windows files NUL, COM0, COM1, etc.

/etc is system-wide configuration.

/media is for storage media. It is like "My Computer".

/mnt is used as a temporary mount moint where putting in it /media will clutter things up.

/opt is for third-party binary packages. It is like C:\Program Files

/root is the Administator (root)'s home directory

/proc is a virtual filesystem with files detailing the operation of the system, some of which you can change.

/sbin is for essential administration programs

/sys is like proc but newer (and different).