Scholarly or Academic: Written by academic experts (often PhDs) in a field, for other experts.

Peer-Reviewed or Refereed: Sent out to other experts in the field for feedback before publication. Many articles are rejected at this stage, or substantially revised. Those that make it through have been therefore been approved by multiple experts.

Trade magazine/journal: Written by and for professionals in the field, but not necessarily scholars, and usually focused on practical issues. 

Popular magazine: Written by journalists and professional writers for the purposes of entertainment and general information.

If you find a scholarly article in your research, you may find it more difficult to get through than articles in trade and popular magazines. It can help to break the article up into sections and read strategically rather than straight through. In many disciplines, there are standard section types you can expect, which can also help guide your reading.