What do these abbreviations stand for?

Strangely enough /usr actually means Unix System Resources.

"The "etc" in "/etc/bin" really does stand for "etcetera." In early Unix systems, the most important directory was the "bin" directory (short for "binaries" -- compiled programs), and "etc" was for trivial stuff like startup, shutdown and admin. The list of things you need for running Linux is: a program binary, etcetera, etcetera -- in other words, a sole vital item, plus some less important bits and pieces.

Today, "etc" holds system-wide configuration files that you'd almost never do without -- hardly unimportant." --http://searchenterpriselinux.techtarget.com/tip/0,289483,sid39_gci1098161,00.html

You might find the output from "man hier" fascinating

for the curious reader, sbin is short for “system binaries” (although it’s sometimes been described as being short for “static binaries”, because on some systems the executables in /sbin would always be statically-linked to ensure a bootable system if /lib was destroyed); lib, fairly obviously, stands for “libraries”.

/var contains “variable files” — things like logs, process IDs, temporary directories, mail spools.

/proc (on systems which have a proc filesystem) originally just contained information about running processes, but Linux extended this to include lots of other information, too (for example, cat /proc/cpuinfo).

On some older Unix variants, /etc often contained executable programs (and actually may still do, in some cases), rather than simply being restricted to configuration files. If memory serves, ifconfig was actually /etc/ifconfig on SVR4 systems some years ago.

If you use a Solaris machine, you’ll also see xpg4 (standing for “X/Open Portability Guide”), ccs (“C Compiler System”), and ucb (“University of California, Berkeley” — BSD compatibity).

You may find reading up on the FHS helpful :)

/etc stands for et cetera. Wikipedia references a Bell Labs document from '72 that calls it etcetera.

The Filesystem Hierarchy Standard has information on etc history:

http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard

There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell labs, /etc is referred to as the etcetera directory,[24] as this directory historically held everything that did not belong elsewhere (however, the FHS restricts /etc to static configuration files and may not contain binaries). Since the publication of early documentation, the directory name has been re-designated in various ways. Recent interpretations include backronyms such as "Editable Text Configuration" or "Extended Tool Chest".