The peer review process tries to ensure that only the highest quality research is published. When an article is submitted to a peer-reviewed (or refereed) journal, the editor sends it to be reviewed by other scholars (the author's peers) in the same field. These reviewers recommend that the editor reject the paper, accept the paper as is, or accept the paper with author revisions.
Though trustworthy, peer review is not without flaws. The Retraction Watch blog tracks when published articles are withdrawn due to plagiarism, error, fraud, or other reasons.