Taken from attenboroughp's Linux handbook:
Catagories for programs:
bin - programs (usually binary files)
sbin - programs (usually binary files) intended to be run by root (single-user binaries)
lib - libraries of code used by programs
libexec - programs invoked by other programs
usr - UNIX System Resources (NOT user)

Catagories for documentation
doc - documentation
info - documentation files for Emac's built in help system
man - manual pages (for man command)
share - program specific files, such as examples and installation instructions

Catagories for configuration:
etc - configuration files for the system
init.d - configuration files for booting linux; also rc1.d, rc2.d, ...

Catagories for programming:
include - header files for programming
src - source code for programming

Catagories for web files:
cgi-bin - scrips/programmes that run on web pages
html - web pages
public_html - web pages, typically in users home directories
www - web pages

Catagories for display:
fonts - fonts!
X11 - X windows system files

Catagories for hardware:
dev - device files for interacting with disks and other hardware
mnt - mount points: directories that provide access to disks

catagories for runtime files:
var - files specific to this computer, created and updated as the computer runs
lock - lock files, created by files to say "I am running"
log - log files that track important system events. Errors ect...
mail - mailboxes
run - PID (Process ID) files, which contain the ID's of running processes
spool - files queued in transit e.g outgoing mail, print jobs...
tmp - tempory storage (/tmp is usually stored in RAM, /var/tmp is temp files on the disk. /tmp will be cleared on restart)
proc - operating system state

Taken from my Solaris admin book (may be different from Linux):
/bin - Link to /usr/bin hold binaries (programs)
/dev - Logical devices (files in here link to /devices)
/etc - Host-specific configuration file and databases for system administration
/export - Directory for commonly shared file systems such as users home directories, application software, etc
/home - The default directory to mount users directories in
/kernel - Platform-independent kernel modules required during boot
/lib - Shared executable files (libraries)
/mnt - Convenient mount point for temporary file systems (CD's/USB drives,etc)
/opt - Default directory for add-on software
/platform - Platform-independent kernel modules
/sbin - Single-user binaries
/usr - Contains programs, scripts and libraries used by all system users.
/var - Directory for varying files: temporary, logging and status files for example.
/tmp - File system in memory, anything put here will have disappeared after a shutdown/restart
/usr/include - C header files etc
/var/adm - Syslog (messages) and other log files
/var/spool - Spooled files such as mail, print services and cron jobs
/var/tmp - Long term temporary storage, preserved over reboots