According to the Merriam-Webster Online Dictionary, to "plagiarize" means

• to steal and pass off (the ideas or words of another) as one's own
• to use (another's production) without crediting the source
• to commit literary theft
• to present as new and original an idea or product derived from an existing source.

In other words, plagiarism is an act of fraud. It involves both stealing someone else's work and lying about it afterward.

When do I need to cite?

Whenever you borrow words or ideas, you need to acknowledge their source. The following situations almost always require citation:

1. whenever you use quotes
2. whenever you paraphrase
3. whenever you use an idea that someone else has already expressed
4. whenever you make specific reference to the work of another
5. whenever someone else's work has been critical in developing your own ideas.

What is citation?

A "citation" is the way you tell your readers that certain material in your work came from another source. It also gives your readers the information necessary to find that source again, including:

1. information about the author
2. the title of the work
3. the name and location of the company that published your copy of the source
4. the date your copy was published
5. the page numbers of the material you are borrowing

http://www.plagiarism.org/

What is Plagiarism? (n.d.) Retrieved August 15, 2010, from <https://www.plagiarism.org/article/what-is-plagiarism>

Purdue Online Writing Lab (OWL) APA Formatting and Style Guide, 6th Edition