Disclaimer: I am not a lawyer.
If you want to post content from a single answer or a single question, you need to contact the person who posted it as they own the rights to that content.
If you want to post a question/answer combination then you have to follow the terms written in the Stack Exchange Terms of Service.
Individual submissions ('posts') to the Stack Exchange Network (SE) are owned by the person who created them. Individual submissions are licensed only to SE and any reposting elsewhere by non-SE entities, regardless of attribution, are grounds for issuing a DMCA takedown notice.
Posts are licensed to SE to use in a Collection/Collective Work which is a new creation that SE holds the rights to. If a Collection/Collective Work (for instance a question and answer combination) is used without following the attribution rules from the SE Terms of Service (TOS), then SE is able to issue a DMCA takedown notice for that content.
General Overview of the License
Content is owned by the content creator. The content creator licenses that content to SE under the terms of the Stack Exchange TOS with the following restrictions:
- SE can use the content in all sorts of ways even if the Subscriber decide to delete the content later1, including the right to claim copyright over the entire content of the Stack Exchange Network as a collective work/compilation2
- The Subscriber is responsible for making sure SE can use the content in all those ways3
1: "You grant Stack Exchange the perpetual and irrevocable right and license to use, copy, cache, publish, display, distribute, modify, create derivative works and store such Subscriber Content and to allow others to do so in any medium now known or hereinafter developed (“Content License”) in order to provide the Services, even if such Subscriber Content has been contributed and subsequently removed by You."
2: "The Network is protected by copyright as a collective work and/or compilation, pursuant to U.S. copyright laws, international conventions, and other copyright laws."
3: "Subscriber warrants, represents and agrees Subscriber has the right to grant Stack Exchange and the Network the rights set forth above."
SE combines questions and answers creating a Collection or a Collective Work which they own the rights to. They own the rights to that Collection and re-license it with the following restrictions:
In the event that You post or otherwise use Subscriber Content outside of the Network or Services, whether such Subscriber Content was created by You or others, You agree that You will follow the attribution rules of the Creative Commons Attribution Share Alike license as follows:
a) You will ensure that any such use of Subscriber Content visually displays or otherwise indicates the source of the Subscriber Content as coming from the Stack Exchange Network. This requirement is satisfied with a discreet text blurb, or some other unobtrusive but clear visual indication.
b) You will ensure that any such Internet use of Subscriber Content includes a hyperlink directly to the original question on the source site on the Network (e.g., http://stackoverflow.com/questions/12345)
c) You will ensure that any such use of Subscriber Content visually display or otherwise clearly indicate the author names for every question and answer so used.
Note: There is a question over whether or not the
rel:nofollow is a violation of the CC-BY-SA license or not
So Who Owns the Content?
Despite not owning the copyright to Subscriber Content, SE does own the right to the content as a Collection/Collective Work. As stated on copyright.gov:
Under the present copyright law, the copyright in a separate contribution to a published collective work such as a periodical is distinct from the copyright in the collective work as a whole. In the absence of an express transfer from the author of the individual article, the copyright owner in the collective work is presumed to have acquired only the privilege of using the contribution in the collective work and in subsequent revisions and later editions of the collective work.
This is a normal English explanation of US Title 17 Sections 102-103
This is also covered under the definition of Collection in the CC-BY-SA license:
"Collection" means a collection of literary or artistic works, such as encyclopedias and anthologies, or performances, phonograms or broadcasts, or other works or subject matter other than works listed in Section 1(f) below, which, by reason of the selection and arrangement of their contents, constitute intellectual creations, in which the Work is included in its entirety in unmodified form along with one or more other contributions, each constituting separate and independent works in themselves, which together are assembled into a collective whole. A work that constitutes a Collection will not be considered an Adaptation (as defined below) for the purposes of this License.
This allows SE to manage the rights to the collective work, including issuing takedown notices for content shared through SE even though they are not the owner of the individual works (the Subscriber Content).