What is a Peer Reviewed Journal?

Whether it appears in print, a combination of print and electronic forms, or only in electronic form, a peer reviewed journal is one in which each feature article has been examined by people with credentials in the article's field of study before it is published. 

Collections of papers from conferences may be considered peer reviewed as well, if the original presentations were "invited" or examined by experts before being accepted. Papers which appear in sources like these are considered to be as reliable as humanly possible.

In "double blind" peer review, neither the author nor the reviewers know each others' identities. Not all peer review is double blind. As well, the literature of any profession may include journals written both for practitioners and for theorists.

You can sometimes categorize a journal just by looking at its first and last few pages. Check to see if there is a page listing the Editorial Board, and see what credentials they have.  If there is a page containing "instructions for authors", it may actually say the journal is peer reviewed.  If the instructions mention submitting several copies, with your name on  the front page but not anywhere else in the manuscript, that is your cue that the journal uses double blind peer review.
 

Cover art and layout can be clues to content.
 

Scholarly journals do not usually contain advertisements, except for non-profit events or for other journals by that publisher. That way, they cannot be accused of  commercial bias. Scholarly journals may come out quarterly, bi-monthly, monthly, etc., and are often printed on book sized paper, and have a serious "look".  Journals written for practitioners rather than for theorists often come out on glossy paper, contain more photographs, and include paid advertising.

The Format of a Scholarly Article

In the physical and social sciences, scholarly articles usually follow a standard format, which is a less rigorous version of that used in masters' theses and doctoral dissertations. While some writers and readers may find the format restrictive, it exists for an important reason. It allows other researchers to replicate the work so as to confirm or reject its findings. This notion of replicability is essential to contemporary science.

Articles written in this format usually contain section headings like these:

Abstract - a short summary of the article.

Keywords.

Introduction and Statement of the Problem - identifies the need for the work, and poses the research question. (For example, "Do people use the library more or less, now that there is an Internet?").

Review of the Literature - places the work in context.

Restatement of the Research Question as an Hypothesis - In the most rigorous research, the research question is expressed as an hypothesis, using dependent and independent variables, which can be supported or refuted using appropriate statistical methods. (For example: Happiness (H) will be directly proportional to consumption of ice-cream (C_i)".

Methodology - explains the methods so others can replicate the study.

Data Collection - describes the process and points out potential omissions.

Analysis - examines the data by qualitative or quantitative means, states whether the research question or hypothesis was proven or disproven.

Conclusions and Recommendations for Further Research - accounts for the results, suggests explanations, points out things that might have been overlooked, and suggests future avenues for research. (For example: "We were only able to survey English, French, and Spanish speakers. The next study will include speakers of aboriginal languages.")

References - clear citations, which lead to other people's works that were mentioned in the article.

Problems With Peer Review

No researcher can claim to present "the unvarnished Truth" or "the definitive study". Other scholars must corroborate or refute any work presented. The vetting process takes a relatively long time; perhaps up to a year.However, since most journals now make use of e-mail or online systems, the process can be much quicker. Some people point out that, in very small fields, it is likely that reviewers may guess the identities of authors when they examine the manuscripts. Sometimes, if a scholar advances an unpopular or a truly revolutionary point of view, there may be considerable resistance within the peer group. Nevertheless, peer review is the best mechanism we have to attain scholarly objectivity and to guard against scientific fraud.