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