You will encounter many types of articles and it is important to distinguish between these different categories of scholarly literature. Keep in mind the following definitions.
Peer-reviewed (or refereed): Refers to articles that have undergone a rigorous review process, often including revisions to the original manuscript, by peers in their discipline, before publication in a scholarly journal. This can include empirical studies, review articles, meta-analyses among others.
Empirical study (or primary article): An empirical study is one that aims to gain new knowledge on a topic through direct or indirect observation and research. These include quantitative or qualitative data and analysis. In science, an empirical article will often include the following sections: Introduction, Methods, Results, and Discussion.
Review article: In the scientific literature, this is a type of article that provides a synthesis of existing research on a particular topic. These are useful when you want to get an idea of a body of research that you are not yet familiar with. It differs from a systematic review in that it does not aim to capture ALL of the research on a particular topic.
Systematic review: This is a methodical and thorough literature review focused on a particular research question. It's aim is to identify and synthesize all of the scholarly research on a particular topic in an unbiased, reproducible way to provide evidence for practice and policy-making. It may involve a meta-analysis (see below).
Meta-analysis: This is a type of research study that combines or contrasts data from different independent studies in a new analysis in order to strengthen the understanding of a particular topic. There are many methods, some complex, applied to performing this type of analysis.
TIP: Review articles and Peer-reviewed articles are not the same thing! Review articles synthesize and analyze the results of multiple studies on a topic; peer-reviewed articles are articles of any kind that have been vetted for quality by an expert or number of experts in the field. A review article does not qualify as one of your four required articles. However, review articles can be included in your bibliography if they were a useful reference during your writing process and the bibliographies of review articles can be a great source of original, peer-reviewed empirical articles.
There are a couple of ways you can tell if a journal is peer-reviewed:
Using Ulrich's Periodicals Directory to Verify Peer Review of Your Journal Title
To use this, click on the Ulrich's link to enter the database (or search for it on the main library website).
This will give you a list of journal titles which includes the title you typed in. Check the Legend in the upper right corner to view the Refereed symbol ("refereed" is another term for peer-reviewed.) Then check your journal title to make sure it has the refereed symbol next to it.
NOTE: Though a journal can be peer-reviewed, letters to the editor and news reports in those same scientific journals are not! Make sure your article is a primary research article.
They both come out once every month. They're both in English. Both published in the United States. Both of them are "factual".They both have pictures. They even cover some of the same topics.
The difference is that one--Oceanography--is peer-reviewed, whereas National Geographic is a popular-press title.
Peer review is scientists' and other scholars' best effort to publish accurate information. Each article has been submitted by a researcher, and then reviewed by other scholars in the same field to ensure that it is sound science. What they are looking for is that:
It isn't a perfect system: Scientists make errors (or commit fraud) as often as any other human being and sometimes bad articles slip through. But in general, peer-review ensures that many trained eyes have seen an article before it appears in print.
Peer-reviewed journals are generally considered "primary source" material: When a new scientific discovery is made, a peer-reviewed journal is often--but not always--the first place it appears.
Popular and trade publications are not peer-reviewed, they are simply edited. That does not mean they are any less potentially truthful or informative--most popular and trade publications take pride in careful fact-checking.* But when the topic is scientific research, the information is generally "secondary": It has already appeared elsewhere (usually in a peer-reviewed journal) and has now been "digested" for a broader audience.
Peer-reviewed journals will always identify themselves as such. If you want to verify that a journal is peer-reviewed, check Ulrich's Periodical Directory.
Some sources of peer-reviewed articles: