There are two types of scholarly journal articles that you are likely to come across, research articles (sometimes called primary sources) and review articles. Both of these types of articles are peer-reviewed, and both types tend to look the same at first glance: they have an abstract and introduction, a concluding section, and a bibliography.
Both research and review articles are usually published in what are referred to as peer-reviewed journals. You might also hear the term "scholarly journal" or "refereed journal", these all refer to the same thing.
The goal of peer review is to have other experts (reviewers) assess the quality of the research and the article before it is published. During the peer-review process, reviewers will:
The peer-review process is intensive and lengthy. It means that new research can take a while before it's published, but it also provides an assurance of best research practices.
Research articles are generally the main way that new research results and interpretations are communicated in the field of geology. Research articles will focus on one study to answer a research question. These articles tend to be very narrowly-focused and written for geologists in that particular field. Research articles are usually published in scholarly journals, but are sometimes collected in edited volumes (see below).
Some of the main features of a research article are:
Example:
Grasemann, B. and Stüwe, K., 2001, The development of flanking folds during simple shear and their use as kinematic indicators:
Journal of Structural Geology, v. 23, p. 715-725, doi:10.1016/S0191-8141(00)00108-5.
Notice in the abstract, they start off explaining the modeling methods they use in the study. They describe a narrow question and how they answered it. This is indicative of a research article where they are reporting their original results of research.
Review articles, sometimes called literature reviews, pull together the research and conclusion of many research articles on the same topic. The review article synthesizes and analyzes those studies and summarizes the current state of research and suggests directions for future reearch.
Review articles are a great way to get a better understanding of the existing research on a topic, identify research questions you would like to explore and find relevant sources.
In review articles:
Example:
Hudleston, P.J. and Treagus, S.H., 2010, Information from folds: A review: Journal of Structural Geology, v. 32, p. 2042-2071, doi:10.1016/j.jsg.2010.08.011.
In this review article, you can see a few clues that it is a review: the title has the word "review" in it, the topic is relatively broad, and the abstract talks about reviewing the topic.
Gray literature is kind of a "catch all" term that refers to types of research that are either unpublished or have been published in noncommercial forms. While some gray literature is available on the web, be aware that some may be hard to get a hold of.
Gray literature can be very important in geology, so it's a good idea to understand what is out there and how to find it.
Examples of gray literature include:
A lot of geologic information is shared on websites. When you're looking for websites, you'll want to think critically about who is responsible for that website and whether that makes them a credible source. This isn't always easy as some websites don't share that information, so you may need to dig a bit.
Generally, government agencies that are responsible for geologic information, professional geologic societies, and geology departments at universities are considered credible sources.
You can categorize scholarly geology books into two main categories: edited volumes and monographs.
These books function more as collections of research articles around a single topic. You will notice that the volume will have an editor or editors, and each chapter is written by a different group of authors. The editors are experts in the topic who organize but do not write each chapter. Each chapter will look like a research article that you would find in a journal, with an abstract, methodology, results, conclusions, and a bibliography. Many of these undergo peer-review.
Example:
Sediments, Morphology and Sedimentary Processes on Continental Shelves by 
Here is what the first page of one of the chapters looks like, it looks almost exactly like a journal article, there is an abstract and the Introduction section. If you continue to read the article, you'll find method, conclusions and a lengthy bibliography:
Monographs are full books written on a topic, usually with one or just a few authors. Unlike the edited volumes that pull together related works all written by different people, the chapters in the monograph are all written by the same authors and are generally meant to be read as a whole. The authors are experts in the area, but they're not reporting new research results. Instead, they are providing a more comprehensive overview of the topic. Books like these can give you a good broad understanding of a topic, but they will not have the latest research because monographs can take quite a long time to publish.
Example:
Geology and Sedimentology of the Korean Peninsula by 
Here is what the first page of one of the chapters in this book looks like. It has a chapter title and section headings, but no author listed (becuase the author Sung Kwun Chough has written all the chapters of this book) and no abstract. If you continued to read this chapter, you will find information about phanerozoic granites, but not a report of a specific research study. There are no results or conclusions or bibliography.