Plagiarism
Plagiarism – Using someone else’s ideas or words without giving him/her proper credit.
Plagiarism is wrong; it is a form of theft (you are taking someone else's ideas and using them as your own - that's stealing!), and it is illegal.
If you are caught plagiarizing, intentionally or not, the least punishment you will probably receive is a zero for an assignment; many schools, especially colleges, will expel you. You could also be sued by the original author.
(In other words, give credit where credit is due!)
Plagiarism is wrong whether the information is copyrighted or in the public domain.
Information on MLA formatting for citations and bibliographies (so you can avoid plagiarism)