Licenses acts in the FLOSS community almost like a constitution. It gives the fundamental “rules of the game”. It is through licenses the basic principles of FLOSS is maintained. There exist many different licenses that claims to be FLOSS. In an effort to state some formal principles on what constitute FLOSS the OSI www.opensource.org have made the Open Source Definition. Based on this definition OSI offers certification of licenses to help people distinguish between what is and what is not a FLOSS license.
Those licenses that are approved by OSI can be divided into two categories. The first is the GPL style license and the second is the BSD style license. GPL is an acronym for General Public License and is one of the most used licenses. The GPL license was made by FSF in order to make free software possible within the current copyright regime. In the GPL license the author gives explicit rights to copy, modify and distribute software. In addition it requires software derived from GPL code to be licensed with GPL. This requirement is often called the “viral” clause, particularly by its opponents.
The purpose of the “viral” clause is to stop anybody from taking free software, modify it and release it under a proprietary license. When Richard Stallman was working on a Lisp interpreter a company called Symbolics asked to use his interpreter. Stallman gave them a public domain version of his interpreter. Symbolics extended the Lisp interpreter. When later Stallman wanted to access this extensions he was refused. The primary reason behind the GPL is to prevent this kind of behaviour. FSF designed the GPL to make sure that once software is made free it stays free.
The BSD kind of licenses is similar to the GPL style license, it allows for modification and distribution, except it does not have the “viral” clause. The BSD license in it self is made by Berkeley University and is very permissive. It basically says that you can do what you want with it, but use it at your own risk.
Weber (2004) interprets this to mean that the difference between this two styles of licenses are based on different interest in the software. The GPL style license are based on a long term interest in the software, it’s intention is to keep the software free during the whole life cycle of the software. It is based on a philosophy that software should be free (as in free speech) and therefor the license must secure that the software stays free. The BSD style licenses is made by an university which is often not interested in the software on a long term basis. After a research project is finished the researchers wants to move on to other projects. The BSD style license is also preferred by many in the business world because it makes it legal to make proprietary derivation of the work.
A variation on the GPL style license is whether the license allows proprietary software to link with the licensed software. This allows proprietary software to use the licensed software through calling it’s functions/methods, extending its classes, implementing its interfaces and similar methods of linking to the software. Extension to the software itself must be released under the same license. The original GPL do not allow this, but the Lesser General Public License (LGPL) do. The intension is to spread the use of the licensed software, even as part of proprietary software, but at the same time to ensure that the software itself stays free. GNU calls this license Lesser because it only encourages it to be used for libraries, databases etc., where there already exists proprietary alternatives. For a figure on the different categories of software licenses see figure 5.1.