Your research paper is a collaboration between you and your sources. To be fair and ethical, you must acknowledge your debt to the writers of those sources. If you don’t, you commit plagiarism, a serious academic offense.
In general, these three acts are considered plagiarism: (1) failing to cite quotations and borrowed ideas, (2) failing to enclose borrowed language in quotation marks, and (3) failing to put summaries and paraphrases in your own words. Definitions of plagiarism may vary; it’s a good idea to find out how your school defines academic dishonesty.