Popular journals are often called magazines. These magazines may contain news and stories about current events, but they often also contain sensational "gossip" and articles. They are written for a broad, general audience and rarely have a place when researching material for an assignment.
Scholarly journals contain articles that have been well-researched, cover a topic in depth and many of the articles will have gone through a peer-review process. If an article has been through peer-review before publication you know you can use this article confidently in any assignment or essay for your teachers.