A primary source is an original document -- first-hand information. Primary source research will include results of prior experiments (Review of Literature) and statistical data. Statistical data, when collected by the researcher or investigator is primary data. In the natural and social sciences, primary sources are often empirical studies -- research where an original experiment was done or a direct observation was made.
The results of empirical studies are typically found in scholarly articles or papers delivered at conferences, so those articles and papers that present the original results are considered primary sources.
What is Peer Review? A process by which a scholarly work (such as a paper or a research experiment or proposal) is checked by a group of experts (peers) in the same field to make sure it meets the necessary standards before it is published. In a database, narrowing your search by checking "peer reviewed" or "scholarly" is encouraged, since it can help to locate a valid primary source of research.
Secondary sources analyze, review or restate information in primary resources or other secondary resources. Even sources presenting facts or descriptions about events are secondary unless they are based on direct participation or observation. Moreover, secondary sources often rely on other secondary sources and standard disciplinary methods to reach results, and they provide the principle sources of analysis about primary sources. Examples include: