"Peer review is the critical assessment of manuscripts submitted to journals by experts who are usually not part of the editorial staff."
Additionally: "Because unbiased, independent, critical assessment is an intrinsic part of all scholarly work, including scientific research, peer review is an important extension of the scientific process."
Sometimes we hear a journal described as "peer reviewed" but papers are peer reviewed and not a complete journal. Non-peer-reviewed editorials and news updates may appear alongside articles which are themselves reviewed.
For help in determining if a paper has gone through peer review:
Look carefully at the notes with a paper for a description of its progress through the process, often including date received and date finalized for publication.
Check a journal's or conference's website for author information about how papers are reviews.
Use the guidance of the library database: Ulrichsweb.
Use the guidance of library databases which index articles and label them as, for example, "scholarly."