Primary sources are those where a researcher can find original data.
The major source of primary information are peer-reviewed journals.
Popular Press: Material written for the general public.
Scholarly Journal: Articles written by experts in academic or professional fields.
Peer-Reviewed: Articles are submitted to rigorous scrutiny by other subject experts prior to publication.
Reference Books are a great way to acquaint yourself with a new topic, find researcher or sources in a given field, and learn terminology.
General encyclopedias are not appropriate to cite in your paper.