License of GNU Emacs.
 GPL in a nutshell
- Someone (called A) creates a recipe of beer and publishes it under the terms of the GPL
- here is what another guy (called B) can (and/or has to) do
- B can make beer, made with this recipe
- B can make money by selling beer made with the recipe
- B can improve the recipe, and publish it
- B can tell the recipe to others
- B can improve the recipe, but must publish it under the terms of the GPL
- when someone (called C) asks B, what the original recipe is, B has to tell him
- when C asks B, what the improved recipe is, B has to tell him
- in one sentence, when A creates a recipe of beer, publishes it under the terms of the GPL, recipe and improved recipes will be publicly accessable forever, and anyone can make money by making and selling the beer and/or distributing the recipe.