Research articles (which your professor may also call peer-reviewed, refereed, or academic articles) are considered the most reliable information sources. They are written by experts and go through a rigorous process where other subject experts analyze and critique the arguments to identify errors, faulty logic, and other problems.
News Articles are usually considered primary sources and are excellent for finding information about an event around the time that it occurred. They are not peer-reviewed or scholarly in nature.
Legal Research involves finding the legislation, court cases, and law review discussions that make up the American judicial system.
Government Resources link you to government websites for Political Science topics.
Research Centers provide research reports and opinion pieces. These may be biased, so adopt concepts and research with care to cross check and verify information.