Source: http://plagiarism.org/plagiarism-101/what-is-plagiarism
WHAT IS PLAGIARISM?
Many people think of plagiarism as copying another's work or borrowing someone else's original ideas. But terms like "copying" and "borrowing" can disguise the seriousness of the offense:
ACCORDING TO THE MERRIAM-WEBSTER ONLINE DICTIONARY, TO "PLAGIARIZE" MEANS
In other words, plagiarism is an act of fraud. It involves both stealing someone else's work and lying about it afterward.
BUT CAN WORDS AND IDEAS REALLY BE STOLEN?
The expression of original ideas is considered intellectual property and is protected by copyright laws, just like original inventions. Almost all forms of expression fall under copyright protection as long as they are recorded in some way (such as a book or a computer file).
ALL OF THE FOLLOWING ARE CONSIDERED PLAGIARISM:
Most cases of plagiarism can be avoided, however, by citing sources. Simply acknowledging that certain material has been borrowed and providing your audience with the information necessary to find that source is usually enough to prevent plagiarism. See our section on citation for more information on how to cite sources properly.