Primary resources --peer reviewed articles written by authors who actually performed an original experiment or are reporting their field observations, i.e., of organisms or medical patients.

Secondary resources --peer reviewed articles written by authors who summarize or discuss trends in the primary literature.  In the life sciences, these are called "review" articles or "reviews."

Tertiary resources --non-peer reviewed articles, books, newspaper articles, encyclopedia entries, etc.. 

Peer review--process by which articles are submitted to experts in the scientific discipline for comments and criticism before publication in a scholarly journal. Peer reviewed journals are also called "refereed."

Find out if a publication is peer reviewed by looking for it in this database. If the publication has a referee jersey next to it, it is peer reviewed.

Citation data is a numerical tool for evaluation of individual articles.  In general, the more an article is cited in the reference lists of other articles, the more impact the article has had on the scientific community.  

Watch this tutorial to understand what peer review is and how to determine if a journal is peer reviewed.