Magazines are periodicals. Newspapers are periodicals. Why do librarians call them periodicals? It's our business to collect and provide access to all kinds of publications. We use the word "periodical" to distinguish publications that are issued periodically--daily, weekly, monthly, quarterly--from publications that are issued singly, like books.
Librarians and publishers call newspapers and magazines "popular" periodicals.
Journals are also often called academic or scholarly to indicate both their origins (often in academic institutions) and audiences (often scholars). They are also often peer-reviewed or refereed.
People new to these terms often confuse journal with article. An article is a single authored item within any kind of periodical. A journal is a periodically-published collection of articles by different authors.
Many academic journals are peer-reviewed or refereed journals. When an author (usually a university professor and/or researcher) submits an article, copies are sent to several reviewers who are expert scholars in the author's field. These reviewers then recommend whether or not the article should be published. Some journals reject as many as 90% of submissions. This rigrorous review process provides a high degree of credibility.
Two short, informative videos about Scholarly and Peer-Reviewed journals:
What is a Scholarly Journal Article? (3 minutes)
Though peer-reviewed, refereed, scholarly, and academic are not exactly the same thing, they are often used interchangeably.