One of the most important aspects of Wikipedia is that its text (not media, but that will be discussed shortly) may be freely redistributed, reused and built upon by anyone, under the terms of the (CC BY-SA) and, except where otherwise noted, the GNU Free Documentation License (GFDL) (unversioned, with no invariant sections, front-cover texts, or back-cover texts). Contributors agree to release their original content under both licenses when they submit it, and material from public domain sources or other compatibly licensed sources may also be used in accordance with the copyright policy, provided correct attribution is given.
However, copying material without the permission of the copyright holder from sources that are not public domain or compatibly licensed (unless it's a brief quotation used in accordance with Wikipedia's non-free content policy and guideline) is likely to be a copyright violation. Even inserting text copied with some changes can be a copyright violation if there is substantial linguistic similarity in creative language or sentence structure; this is known as close paraphrasing, which can also raise concerns about plagiarism. Such a situation should be treated seriously, as copyright violations not only harm Wikipedia's redistributability, but also create legal issues.
The situation for images and other media is slightly different, as a wider variety of licenses is accepted. But, in short, media which is not available under a suitable free license and which does not meet the non-free content criteria, should be assumed to be unacceptable. See Wikipedia:Image use policy and Wikipedia:Non-free content for details of this and Wikipedia:Guide to image deletion for some suggested steps for handling problems with images or other files.
If a contributor has already been clearly warned of copyright infringement but carried on, you may want to seek advice from an administrator familiar with copyright policies or report it for administrator attention at the administrators' incidents noticeboard. You may also want to open a request at Wikipedia:Contributor copyright investigations.
Contributors who repeatedly post copyrighted material despite appropriate warning may be blocked from editing by any administrator to prevent further problems. Contributors who have extensively violated copyright policy by uploading many copyrighted files or placing copyrighted text into numerous articles may be blocked without warning for the protection of the project. These blocks are usually indefinite, pending satisfactory assurances that infringement will not continue. In extreme cases administrators may impose conditions before unblocking, such as helping with cleanup by disclosing the sources they used. If contributors have been shown to have a history of extensive copyright violation, it may be assumed that all of their major contributions are likely to be copyright violations, and they may be removed indiscriminately. See Wikipedia:Contributor copyright investigations.
If you are a copyright owner or represent a copyright owner, and you believe that Wikipedia is infringing your copyright, we can assist you best via e-mail. You may contact info-en-cwikimedia.org with an informal request; please cite the exact URL (the "address" or "location" of the page as shown by your web browser, beginning with ...), and provide enough information to substantiate your claim of copyright ownership. Be aware that correspondence is answered by a small number of volunteers, and an immediate reply may not be possible.
If you are a copyright owner who would like to license their content for reuse, please see Wikipedia:Donating copyrighted materials.