There are several main categories of periodicals. 

Popular Magazines / Newspapers - Often written by journalists or news reporters for a general audience. Magazines and newspapers usually have many photographs and advertisements and cover a wide variety of topics.

Scholarly Journals - Often written by an expert in a very narrow field and reviewed by fellow scholars (peer review). Authors frequently work at a university and/or have an advanced degree such as a Ph.D in their field. Articles in scholarly journals are longer than magazine articles and usually focus on a very narrow topic. These articles frequently report the results of a scientific research study and often include a number of tables or graphs presenting data. You will almost always see a list of references to other research at the end of each scholarly journal article. 

Trade Publications - These publications fall in between popular magazines and scholarly journals. They look like a magazine, but the content is targeted to members of a particular industry or profession. For example, in the library profession we have a magazine called American Libraries.

See the file (Magazines vs. Scholarly Journals) below for a further explanation of these differences.