Books, research articles, and news articles provide different types of information and, especially, different amounts of detail. Which source you need depends on what you want to do with it. You may be looking for a brief overview, a quick fact, or an in-depth analysis. Until you know what you need, it can be difficult to decide where to look for the information.
Primary sources are original works, first-person accounts, and research results. What is considered a primary source differs by discipline and also by how you plan to use it. Some examples include:
Secondary sources summarize or interpret primary sources. These may include review articles, books, news articles, encyclopedias, and text books.
Consider this: an encyclopedia is generally considered a secondary (or even tertiary) source of information. However, if your research is on how the encyclopedias have treated a certain topic over time, then the encyclopedias you consult will be considered primary sources.
Popular sources are often written by journalists. Journalists are good writers and researchers, but they are not necessarily experts on the topic. These sources are written for a general audience.
Scholarly sources are written or compiled by experts for an academic or professional audience. These are often original research on very specific topics. They include bibliographies.
To illustrate the above:
A recent popular piece in The Economist:
Vs. a scholarly article in Applied Cognitive Psychology:
Peer review is a process by which experts in a field read and evaluate a journal article before it is is published to ensure quality. Generally, several experts will read an article and comment on it. The authors may then revise the article based on the feedback. Articles may not always pass the review process. Peer review is not necessary for something to be considered "scholarly," but it is an excellent indicator of a "scholarly" article.
The following video is good overview of the peer review process.