The Literature Review is part of a research paper in which the author provides a historical account of the research on the topic, along with recent developments. While other parts of an paper may cite non-research articles, the Literature Review only cites research articles.
It is also an important way for you to become familiar with the specifics of your topic, including specialized language used, key researchers, and which journals are publishing on the topic.
Research articles are primary sources. Secondary Sources include books or trade papers citing original research, while General Sources include newspapers, magazines, blogs, and Wikipedia.