If you include a piece of text or a theory into your own work, you do so by quoting or paraphrasing.
Quoting: literally copying a passage form a text or someone's words. You only do this when the literal words are essential to support your argument.
Paraphrasing: reproducing someone else's text in your own words. Make sure you formulate the message you want to convey entirely in your own words. Paraphrasing is preferable to quoting, because it allows you to incorporate your own understanding of the text.
Whether you paraphrase or quote, you must refer to the source. You must refer to the original source both in your text and in the bibliography.