In linux, everything can be found under the system root, or '/'. It doesn't matter if you have 5 hard drives with 10 partitions, everything appears as a folder underneath '/'.

Each linux distro is slightly different in it's implementation, but there are some standards:

/home/[username] - All user files contained here. Anything that you've worked on, your firefox/thunderbird/kmail profiles live here.

/etc/ - All configuration files live here. Everything from the apt source lists (/etc/apt/sources.lst), the firehol config (/etc/firehol/firehol.conf) to the X-system config (/etc/X11/xorg.conf).

/bin/ - Essential binary files in the system. Usually small utilities like chmod, cp and rm.

/usr/ - Shareable data (but read only). Tends to be non-essential stuff (not having it won't necessarily kill your system immediately, but probably will make it un-bootable...), software installed on the system, any program documentation.

/var/ - variable stuff.. program caches, libraries, temporary space, printer spool space, and system logs

/sbin/ - system binaries. Without these your system won't boot. Or do much of anything..

/tmp/ - temporary file storage. Wiped regularily.

/dev/ - Devices. In Linux (Unix) almost everything is a file. This includes devices. If you wanted to mount a hardrive, you'd probably mount /dev/sda1/ (or similar)

/lib/ - essential system libraries (drivers/kernel modules)

/mnt/ - a folder for you to mount temporary file systems

/opt/ - sometimes used for program data storage, sometimes used to install programs to

/root/ - home directory for the "root" user. (Root user is the system admin, you can do *ANYTHING* as the root user. If you're logged on as it, use with extreme caution. You won't get warned that you're about to destroy your linux install...)

/proc/ - System process holders & status readouts

/media/ - folder to mount removable drives to

/cdrom/ - folder on which your primary cdrom drive is mounted to

/srv/ - On some linux distributions, this is where all files that get served to people are stored. (So for apache's web-root / filestore, it would be in /srv/www/. In Ubuntu, apache's web root can be found in /var/www/ )

Partially nabbed from my answer to a question on a forum and elaborated.

Some of this came from wikipedia, the rest my head.