Popular sources are written for a general audience by a journalist or writer who may not be an expert on the topic. They often summarize or present information gleaned from other sources, and are intended to entertain, persuade, or inform the general public.
Scholarly sources are written for a specialized audience and appear in journals not accessible to a mass market. They are reporting the results of original research. The authors are usually professors or researchers.
*Remember that while peer-reviewed articles are always scholarly, scholarly sources are not always peer-reviewed. Not all academic journals go through the peer-reviewed process, but are still considered scholarly since they have articles written by experts in that field of study.
Primary sources are original documents, such as interviews, letters, original research findings, works of art, legal documents, and more. They are first hand accounts of an event that has happened, and involves someone who has a direct connection with the event or work.
Secondary sources are a report or analysis of something that has already happened. They are an interpretation of a primary source.
Although newspaper articles are typically secondary sources, some are primary; it can be confusing. If the newspaper article is reporting on a recent event, then it would be considered primary. If it's reporting on either something that has happened in the past, or drawing in information from other sources, it would be secondary.
A newspaper article from 1912 that reports on the sinking of the Titanic would be a primary source since it's an article from the time the event happened.
A newspaper article from 2019 reporting on the results of the presidential election of 2016 would be a secondary event since it's reporting on something that has already happened.