Probably the best resource for learning about words like foo, bar, and baz is the Jargon File (available in print as The New Hackers Dictionary). To take a brief excerpt about from the "foo" entry (the complete entry for foo runs over 2 pages):

foo: /foo/ 1. interj. Term of disgust. 2. (very common) Used very generally as a sample name for absolutely anything, esp. programs and files (esp. scratch files). 3. First on the standard list of metasyntactic variables used in syntax examples. See also bar, baz, qux, quux, corge, grault, garply, waldo, fred, plugh, xyzzy, thud.

The Jargon file goes on from there to list several possible origins including:

• derivitive of the WWII-era Army slang acronym FUBAR
• earliest documented uses were in the "Smokey Stover" comic strip popular in the 1930s, which frequently included the word "foo".
• that hacker usage actually sprang from "FOO, Lampoons and Parody", the title of a comic book first issued in September 1958, a joint project of Charles and Robert Crumb.
• FOO: The first syllable of the sacred chant phrase "FOO MANE PADME HUM." Our first obligation is to keep the foo counters turning.

The Jargon File/New Hackers Dictionary is quite a good read. I never let my copy stray too far from my side.