BSD License vs GPL

The difference, as I understand it:

BSD License:
Provides: Source Code (modifiable as desired)
Cost: Must maintain authorship attributions (authors gain recognition - provides credential currency)
Benefits to Purchasers: Are not required to publish their modifications to the source code - can move from free to proprietary

GPL:
Provides: (Software +) Source Code (modifiable as desired)
Cost: Must maintain authorship attributions (authors gain recognition - provides credential currency) and must publish modifications to source when distributing derivative software (authors gain access to the changes to their code - provides capital back to the authors)
Benefits to Purchasers: Gain access to all derivative works

So essentially, the GPL imposes a 'tax' on anyone modifying the covered source code, forcing them to pay up in the form of publishing their modifications (if it's distributed, that is). This has the effect (desired by the designers) of removing the ability to force payment for use of the modifications - ie, it can not be made proprietary (nor can derivative works).


Notes:
1) IANAL - this does not constitute any form of legal advice on these licenses.
2) This is probably incomplete - see the licenses for full details (note that the GPL - and probably the BSD License as well - has many versions).