Peer-Reviewed - Any article that has gone through a process where other academics are asked to review the article and provide feedback before publication. Articles may be accepted, asked to revise and resubmit, or rejected based on comments from peer-reviewers. There is often a "peer-reviewed" button in databases to help you located this type of articles.
Academic Journal Article - Any article published in an academic journal. There are often article types that are not subject to peer review such as review articles and correspondence. However, all academic articles are written for people who are experts in their field of study.
Popular Article - Any article published in a magazine or newspaper. These are usually geared toward a non-expert audience and are written without academic jargon words.
Some databases focus specifically on academic articles, others contain a mixture of academic articles, popular articles, books, and video sources. To focus your results on academic sources, look for a button that says "Peer-review."