There are many different contexts where you might need to refer to other sources. For example to provide background for and motivate a problem, summarize and critically examine research, compare different researchers’ results with each other or with your own results.
When you want to use other people’s research results you can quote or paraphrase to avoid plagiarizing. A quote is identical to the original text while a paraphrase accounts for or summarizes the original text. In both cases you must name the source of the information by adding a reference. Further on in the guide on the page Citations and citation styles you can read about how to cite sources.
You use quotes or paraphrases to support your own thoughts and arguments not to replace them. It is important to not make a patchwork of borrowed texts but to contribute with your own text. When you weave quotes or paraphrases into your own text it is important that it is clear what are your own comments and what you have got from others.