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."

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.  Look for the "Times Cited" information under each article in Web of Science.  Sort your results by "Times Cited" using the drop down menu at the top right to get a feel for the most important papers on your topic.