1. It is the place where the researcher published their findings first. (i.e., the primary place the data is found). 
2. In science, it tends to be a journal article outlining methodology, data, results, conclusions.
3. It will always have a Literature Cited section.
4. It is the researcher's own words, not summarized by anyone else.
5. It is found in scholarly journals such as Animal Behaviour, Journal of Chemical Physics.

Secondary (review) articles summarize other people's primary literature articles. These are a good place to start to understand a topic however. Examples are the Annual Reviews series, many books in the sciences, or any article that has "review" in the title.

Newspaper articles, Newsweek, etc. are NOT primary literature.
Natural History and Smithsonian are secondary literature (reviews), although scientific in scope.