It depends on the assignment and what you are looking, but in general, you will always want to look at both popular and scholarly articles.

Popular Articles

Newspaper articles are written by reporters and mainly provide news as it is happening.  They also contain opinion pieces (editorials and reviews).  They are not written by experts, themselves.  Most of the time, the same can be said for magazine articles, but magazine articles will usually be a bit more in-depth.  Because magazine articles are written for a general audience, the language and discussion of a particular topic are easy to understand, but usually at a cost that more detailed information could be omitted.  Magazine writers are often not an expert in the field discussed, let alone do not do any research beyond who they interview or what they gather at a moment.  So the information on the topic is often far from complete. 

Scholarly/Peer Reviewed Articles

Scholarly journal articles are written by academic experts with material evaluated by other academic experts in their same fields.   With the exception of book reviews or brief feature articles, the articles in scholarly journals delve into serious research to investigate an issue, and may contain an original experiment, population study, or simply be a rigorous review of the literature on the topic.  Such articles usually provide a Works Cited list at the end to demonstrate the seriousness of research. 

Journal articles are a bit cumbersome to one who is just beginning to learn about the subject, however.  They are written with the assumed knowledge of the audience on the discipline, including the jargon-like language of the article itself.