Sharealike
Share-alike is a required reintegration clause of several parametric licenses in Creative Commons and some monolithic ones like GFDL. It simply means that anyone improving a work must share the improved work with others who wish to use or improve it.
It is a key feature of the GPL too which applies this principle to source code. It is sometimes called the viral license feature - the fact that accepting the license breeds more acceptance of the license. This is considered very desirable by some and very undesirable by others (who started the open source movement to oppose the share-alike requirement. See also problems with free software and open source models.
The most common share-alike licenses used are in open content and in free documentation efforts, which require such licenses by definition:
- the GFDL, strictly share-alike, no license but the GFDL can apply to any improvements, and there are strict rules about what can go in Secondary Sections and Invariant Sections to which asymmetric rules might apply - this license is used at Wikipedia
- the CC-by-sa license, quite close to the GFDL but is incompatible with it. Things licensed under the CC regime are not part of the GFDL Corpus. This license is used at Wikitravel
Most efforts use share alike licenses for both code and content, e.g. the CC-nc-sa license - the closest approximation yet to a Green Documentation License - is used for GetWiki code.
See w:Share_Alike for a more general introduction linked to other legal topics.
factional comment:
Reds prefer Free Software apparently because everything "has to be free".
Golds prefer Open Source apparently so they can exploit the commons without obligation to give back.
Trolls like sharing, but bite you on the leg if you try to tell them they have to share with you, even if you are doing harm to them.