Last night Scott Henderson did in impromptu presentation of GNU (a recursive acronym for “GNU’s Not Unix.”) and GPL (General Public License) and the four freedoms of open source software:
The freedom to run the program as you wish, for any purpose.
The freedom to study how the program works, and adapt it to your needs. Access to the source code is a precondition for this.
The freedom to redistribute copies so you can help your neighbor.
The freedom to improve the program, and release your improvements to the public, so that the whole community benefits.