PubMed comprises more than 37 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites. MEDLINE (Medical Literature Analysis and Retrieval System Online) is the largest component of PubMed and consists primarily of journal citations; articles indexed with MeSH (Medical Subject Headings) and curated with funding, genetic, chemical and other metadata.
Research databases index, describe, and add subject terms to articles-- all of which makes it easier to find relevant material. Look for Subject Terms or Descriptors or MeSH terms in the database records, which you can click on or search for to narrow your search to articles primarily on your topic.
Search engines like Google Scholar don't have subject terms or descriptors added to records, but they do search full-text, so an unusual word might work to focus a search.
An article in a Peer Review journal (also called a "refereed journal" is one that has been looked at carefully before publication by experts in a scholarly field (the article author's "peers" in the profession) to determine if the article meets standards of research in a given field and allow it to be published. Peer reviewed or refereed is a mark of quality.
Articles that have been peer reviewed are claiming to be presenting original research, although it is no guarantee that the article is only presenting valid claims.
APA PsycINFO and PubMed are very large databases covering a fixed number of published sources, mainly from peer reviewed publications. Google Scholar is a search engine with potentially infinite coverage -- but not at all limited to peer reviewed or even published, just sources related to an academic publisher or location.
You might be better off starting your searches in databases that are limited almost entirely to a select number of peer reviewed journals. For example, the database Web of Science (which includes Social Science Citation Index and Science Citation Index) is good for this purpose.
There are other things that appear in peer reviewed journals that aren't considered research, such as editorials, letters to the editor, book reviews and announcements. These other things may turn up in a database even when you use a limit filter such as "Peer reviewed." If you have any doubt about something, ask your professor.
Publications that don't use peer review use the judgment of the editor to decide the quality of the article. These are usually only reporting on research done by researchers rather than the researcher formally presenting the research for the first time. Psychology Today and Scientific American are examples of these kinds of publications. These can be great for understanding the intent and significance of the research being reported on.
Web of Science is an excellent tool for tracking down who has used a paper in their own research ("cited" or used as a reference.) Google Scholar has a similar "cited by" link that leads to a much wider (although much less selective) set of material. It is interesting to compare results. The publications in the database Annual Reviews are literature reviews on relatively large topics that put research in context and organize it into themes. Finding an article related to your topic here can be a great way to put your own project in context.
Why cited articles are important: Psychological research is first presented by researchers in academic journals and books. (Sometimes, research with a general interest will get reported in magazines and newspapers and this can be a big help to understanding. See the tab Other Resource Types and Ideas)
But most research grows out of the eye of public awareness. As research interest in a topic grows, there begins to be a group of scholars who begin to connect with each other in a pattern you can see. The pattern emerges in who they cite or refer to in their work.
This citation pattern is called an invisible college and if you are going to write about a topic, you should get some awareness of who these scholars and their works are. Some scholars rise to prominence and have research that most other scholars refer to as the most accepted research. These are the big names you should know.
So, look in two different directions-- at the older foundation works, the most accepted research, and the very new research to see who is being cited most now.
