Giving credit to your sources is an essential part of how scholars communicate. Any time you quote, paraphrase, summarize an author's work--or even if you are just using an idea you didn't invent yourself--you must cite the source with an in-text citation and a more detailed reference at the end so others can find the original sources you've used to construct your own work. For this reason it's very important that you get in the habit of keeping track of your sources as you're gathering material and writing your paper and don't save it for the very end.