Scholarly/Peer-Reviewed
Your sources should be written by scholars, for scholars. This means it should be written by an expert in the field and reviewed and approved by other experts. These types of articles are usually published in scientific or academic journals. Several good examples are listed below in the "Journals" box.
Primary Sources
As graduate students, many of your professors will expect you to be using primary sources. This means that your source should be a "first-person" account. An interview, diary/letter, or more commonly a scientific case study. These can be easily found in a number of the library's databases by using the "limiters" to filter out all other types of sources.