*nix Directories

Root *nix Directories:

 Core:

/bin - compiled ready to run programs
/dev - Devices files (disks)
/etc - System configuration files
/home - Personal directories
/lib -  Library - hold library files used by executables. (note: /usr/lib also exists)
/proc - System stats, information about running processes.
/sys - Similar to proc (TODO: Update)
/sbin - System executables. Users should NOT have /sbin in their PATH.
/tmp
/usr - Not user files! File layout is similar to '/'. Historic, meant to minimise space for root user.
/var - 'Variables' Programs record runtime info. System logging, caches, user tracking etc.

Other:

/boot - kernal boot loader

/media - Removable drives

/opt - Third party software, often not used.

/usr/:

/usr/ - The same as above

/usr/include - Header files for C compiler

/usr/info - GNU manuals

/usr/local - For admins to install stuff here

/usr/man - man pages

/usr/share - historical from the days of low disk space