Peer-reviewed journals (also called "scholarly" or "academic" journals) contain original research articles within a specific subject area.
Peer-reviewed journal articles -- like this PDF example from Pirate Search -- are typically 10-40 pages in length, are reviewed by other subject experts before begin published, and share many or all of these qualities:
NOTE: Some peer-reviewed journals will publish literature reviews, book reviews, and opinion essays. However, these should not be confused for original research articles! If you're unsure whether you've found an original research article, please contact a librarian for assistance!