The software libre or Free Software movement is the philosophy that software, as a form of communication, needs to flow freely without being owned. It is not a form of property.
The capitalized term Free Software is defined by GNU/FSF and typically denotes the "movement" while free software denotes the software itself. It is also possible to take "free software" as an imperative, i.e. a demand to free it.
To facilitate this freedom of flow, and ability to share changes with others, specific licenses with share-alike terms have evolved. This includes the GNU General Public License and the more recent Creative Commons suite.
The Open Source movement by contrast allows for owning improvements and refusing to share them, but builds on a base of relatively free software to enable cooperation and interchange.
Most "open" licenses do not require share-alike and some make it quite difficult. Much open source software is in use today and the Internet is in general built on it, with many vendors offering major enhancements, some of which they have "protected" and "own" (forbidden in the 'free software' model and quite difficult in any share-alike model if they thereby restrict other users from using those improvements or changes).
Other SourceWatch resources
- http://fsf.org - Free Software Foundation - promotes the concept
- problems with free software and open source models