Theory and practice of free software

I was asked this off-list but thought a cc: to the list was appropriate:

If, as you say, Postgres can't use a GPL'd library, does that mean that
Postgres itself isn't GPL'd? If so, then what is it? I'm having trouble
understanding all of the nuances of open source, GPL, etc.

Postgres is distributed under the BSD license, which is a little bit
different from GPL in the detailed terms of what recipients can and
can't do with the software. In particular BSD does not place a
requirement on a recipient to further redistribute the code. There
are several other popular variants on the theme of free source code.

There is a brief overview of common open-source licenses at
http://www.cs.mu.oz.au/~trd/www-free/license_categories.html.
I have seen a more thorough treatment recently, probably at one
of the big open-source sites like Debian or Cygnus, but I can't
find it right now :-( ... anyone have a better link?

Some other good pages that came up while looking:
http://www.fsf.org/philosophy/categories.html (server badly overloaded)
http://www.debian.org/intro/license_disc.html
http://metalab.unc.edu/pub/Linux/LICENSES/theory.html
http://www.opensource.org/osd.html
http://www.debian.org/social_contract
http://www.fsf.org/philosophy/free-sw.html

For background and historical material about hacking culture, it's
difficult to do better than Eric Raymond's writings. See for example
http://www.tuxedo.org/~esr/writings/homesteading/

regards, tom lane