Documentation (the parenthetical citations and works cited page or footnotes and bibliography) and attribution or signal phrases (the introduction of the author/source in the text of your paper) are both important components of academic writing, and you need to learn how to use both properly to clarify whose ideas are whose and to avoid unintentional plagiarism.
One of the primary functions of documentation is to tell your reader where he/she might go to read for him/herself the sources you quote or paraphrase in your paper. That means your documentation should provide the information necessary to lead a reader to the source and the passage you're citing. And, of course, documentation gives credit to the original author.
Attribution or signal phrases gives more specific credit to the author by naming him/her in the text of your essay. Phrases such as "according to Dr. Fred Stanley, director of cancer research at Harvard Medical School" and "Barbara Ladd, noted Faulkner scholar, argues" indicate not only whose idea you are using, but also why that idea merits attention. In other words, signal phrases can be useful persuasive devices in your writing. In addition, this sort of attribution is necessary to clearly establish the boundary between your ideas and a paraphrase or summary of someone else's ideas. Your professors want to see your mind at work on the topic. If you don't use attribution/signal phrases properly, you may inadvertently give up credit for your own ideas. If your citations and attributions are clear, your own thinking will stand out as distinctively your own so that it can be recognized and rewarded.
Different academic fields use different styles of documentation. In many of your courses, you will be asked to use MLA documentation, a style typical of humanities disciplines. Social scientists often use APA. To discover which style to use in documenting your paper, ask the professor in the class. If he/she requires a documentation style that you've never used, ask him/her where to find the guidelines.
Learn more about the basic guidelines of documenting source material (in regards to the Writing Portfolio).