What is the distinction between "open source software", "freeware", "public domain" and "shared source"?

Open Source Software (→ What is Open Source Software?) is distinguished by its comprehensive utility which includes the permission to modify the software.

With freeware, the usefulness can vary widely. The only prerequisite is that the software is free (see → May open source software be commercial?). The rights holder does not have to grant any rights for development or distribution.

"Public Domain" means free to the public.  The term is particularly relevant in the United States since authors can abandon their property rights and thereby specify if their developments are free to the public.  Public software can then be used by anyone without any restrictions.  In Germany and most other European countries, authors can not abandon their copyright based on considerations of personal rights.  Strictly speaking, public domain software does not exist.  Abandonment of copyright therefore has to be interpreted as a license without obligations for the licensee.  In practice, this only makes a difference in a few cases (such as assigning a name). 

The term "shared source software" was introduced by Microsoft to describe programs where the customer can view the source code.  A wide range of license models were used.  Microsoft now licenses its own software under open source licenses.
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