Here you go:
Raspberry Pi • View topic - Filesystem/boot is the stuff directly concerned with booting.
/dev is the devices (serial ports, hard-drives, etc.)
/sys and /proc are pseudo filesystems. They allow access to the operating system using the file metaphor.
/etc are configuration files
/home is where the user's home directories are
/root is the home directory of the superuser
/var is for variable length files -- mail queues, printer queues and system logs.
/tmp is for temporary files and may be purged regularly.
/sbin is the executable files that are required for very early booting and for repairing a broken installation. This directory is available even if only the boot disk can be accessed.
/bin are the other executable files that are required for booting before networking is available.
/include are the C/C++ header files (.h).
/lib are the libraries (.lib)
/usr are the files that do not need to be accessible until after networking is up.
/usr/local are the files that have been installed locally (these days that means not using apt-get etc.)
There are bin, include, lib and maybe etc directories under /usr and /usr/local.
Probably best to head over to that thread for follow-up questions/comments.