In practice open source licenses are differentiated primarily by means of one license condition which is known under the term "Copyleft" as a counterpart to "Copyright". Copyright basically protects the intellectual property of the author of a work or computer program. The author usually is interested in a strong and comprehensive exclusive right, which entitles him as often as possible to prohibit the misuse of his work or to profit substantially therefrom. In contrast thereto the mentioned license condition ("Copyleft") ensures that the source code of open source software explicitly shall be passed on, whereby it also prescribes which conditions shall be applicable for the re-distribution of the amended version of the licensed open source software. OSS licenses are also classified according to the question whether they are compatible with the GNU General Public License (GPL) or not (see: Free Software Foundation).
Licenses containing a strong copyleft clause undertake the user to publish his amendments to the original work under exactly the same conditions. The use of the software code affected by such license is allowed for further developments. However, the further developed work may only be re-distributed under the same license, wherewith such enhancements compulsory remain free. The most famous example is the GNU General Public License (GPL).
On the other hand there are open source licenses which renounce copyleft conditions. Thus the rights of the users of such software are not significantly restrained. Well-know licenses without copyleft clauses are the Berkeley Software Distribution License (BSD) and the license of the Apache Software Foundation. Someone using and enhancing open source software licensed under a BSD license may distribute his enhancement at his choice with costs or for free and with or without source code.
There are also many licenses indeed containing copyleft clauses, which however only affect the direct enhancement of the licensed software codes and allow the developer to keep his rights concerning his own enhancements. An example is the Mozilla Public License (MPL) according to which the free accessibility to the source code of the original work shall remain ensured, but not however for the additional data of an enhancement not containing any MPL-code. For this reason it is possible for software developers to proprietarily utilise the parts of the software developed by themselves based on a MPL, for example as embedded part of a software distribution, as long as it is ensured that the original MPL-code remains freely accessible.