What is copyright infringement?
As a general matter, copyright infringement occurs when a copyrighted work is reproduced, distributed, performed, publicly displayed, or made into a derivative work without the permission of the copyright owner.
Material copied from sources that are not public domain or compatibly licensed without the permission of the copyright holder (unless brief quotation used in accordance with 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's substantial linguistic similarity in creative language or structure (this can also raise problems of plagiarism). Such a situation should be treated seriously, as copyright violations not only harm Wikipedia's redistributability, but also create legal issues.
http://en.wikipedia.org/wiki/Wikipedia:Copyright_violations 
