Plagiarism – Using someone else’s ideas or words without giving him/her proper credit.
Plagiarism is both morally and legally wrong; it is a form of theft (you are taking someone else's ideas and using them as your own - that's stealing!).
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. It's like stealing - even if you only take some of the money that is not yours from the bank, you are still taking money that is not yours from the bank.
Information on MLA formatting for citations and bibliographies (so you can avoid plagiarism)