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 (see below) - 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
See w:Share_Alike for a more general introduction linked to other legal topics.
The term is also sometimes used to imply required reintegration, which is the controversial requirement that open source objects to in both free software and Consortium license software. When this is what is being discussed, it is far better to use this more specific term and not "viral".
Some think they are two separate concerns, but they aren't: required reintegration *is* a part of making a license viral - without it, new software under new licenses can be produced, thus making the original license not apply, and thus not viral, or "as" viral, as the original.
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.