For most people, the most important part of a software license is the end. That's because they're totally uninterested in the license itself; they are merely interested in scrolling past it, so that they can click on "OK" and install the software that they are running. For users of open-source software, however, licenses should be extremely important, because they determine what you may and may not do with a particular program. Open-source developers have long debated the merits of the GNU General Public License vs. the BSD License. As open-source software becomes mainstream, and is integrated into larger systems, these age-old arguments are less theoretical than ever, and should be understood not only by developers, but also by managers and executives of organizations working with open source. Van Lindberg's new book is an excellent place to start.

All open-source licenses are not created equal. Understanding the different licenses is essential before you start to use, and modify, open-source code.

All open-source licenses have certain common elements: They promise that the source code may be used by anyone, for any purposes. They promise that the software may be changed by anyone, in any way that they like. And they promise that people who use the software never have to pay the authors or distributors.

Why are there so many licenses? And how do they differ?