Writers must be credited for their work and their writing. Not to do so is to plagiarize.
Plagiarism is defined as intentionally or unintentionally using the ideas, language, or work of another without acknowledgement that such material is not one's own.
Whenever you quote, paraphrase, summarize, or otherwise refer to the work of another, you are required to cite its source. There are several common systems in use. At Otis, the most common style is MLA (which is short for Modern Language Association), but you may come across others. There are style manuals for each style that you can use.