Scholarly or peer-reviewed articles are written by experts in academic or professional fields. They are excellent sources for finding out what has been studied or researched on a topic as well as to find bibliographies that point to other relevant sources of information.
Primary Sources include original works (diaries, speeches, manuscripts, letters, poetry, art, drama, artifacts) or research (experiments, case studies, interviews, longitudinal studies).
Popular sources are written for the general public and use language easily understood by general readers. They are often written by journalists, and can be good sources for up-to-date, recent information or to assess public opinion.
Secondary sources include critiques, descriptions, or reviews.