The Oxford English Dictionary defines plagiarism as follows:

The action or practice of plagiarizing; the wrongful appropriation or purloining, and publication as one's own, the ideas, or the expression of the ideas (literary, artistic, musical, mechanical, etc.) of another. In other words plagiarism is the borrowing of someone else's creative efforts--words, graphics, music, technical designs (an idea for a machine or device that could be patented). 

Literary plagiarism usually results from the failure to give credit to the author for the use of his/her words or ideas. Failure to do so amounts to theft. Commonly, plagiarism shows that the borrower is either unaware of the borrowing or is too careless to cite the source of the borrowing. It is a simple matter to avoid plagiarism. Basically you must tell the reader in the body or end/foot notes the title, author, and pages of your source. If you borrow from web cites or databases without giving the source, you are still plagiarizing. (See the MLA Citation Section for citing various kinds of sources.) Notice it is not plagiarism to cite information that is considered common knowledge, for example, Martin Luther King’s birth date, nursery rhymes, or folk sayings. 

How to Avoid Plagiarism

Plagiarism often occurs when the writer has not organized his/her ideas around a topic statement and is simply going hunting for material. Taking time to construct a thesis statement makes it is much easier to choose appropriate material and quote others only to support your thesis. Structuring the research in this way allows the writer to keep track of the sources consulted during the construction of the paper.

Penalties for Plagiarism