Literature just means "the stuff that's been published on a topic."
There are three basic “levels” of literature:
Primary Literature: accounts of original research, written by the researchers who performed it. Primary literature must provide enough information to allow another researcher (with the right skills) to reproduce the experiment. Examples: journal articles, conference proceedings, dissertations, and patents.
Secondary Literature: This type of literature refers to the primary sources, in some cases synthesizing information from primary sources to draw new conclusions, or repackaging it in a new form. Generally, secondary literature provides some new information or criticism on the topics discussed. Examples: monographs (a fancy word for books), encyclopedias, some textbooks, and annual reviews.
Tertiary Literature: Tertiary literature is primarily used as a finding aid for primary and secondary literature, and does not usually provide any new information. Examples: directories, indexes, and bibliographies
In terms of importance for scientific research, the most important type is primary literature. Secondary and tertiary literature are there to assist you in locating and understanding the primary literature, but it is the first-hand accounts of actual research that let you know what has been done, and what researchers have discovered.
When beginning research into the scientific literature, it is helpful to know that scientific information follows a fairly well-understood "communication lifecycle," and you can use this knowledge to get an understanding of what kind of information you are likely to find about any given topic.
The "Lifecycle of Scientific Information" begins with an initial research idea - "what if?" or "why does that happen?" From that point, experiments explore the idea and the hypotheses developed by the investigator. Discussion with colleagues and presentations at conferences follow the initial results, and pre-print articles may be archived in digital repositories or published in "letters journals" or preprint journals.
Following the preprint and conference stage, articles are submitted for publication in scholarly, peer-reviewed journals. These articles typically undergo rigorous review by experts in the field before being accepted for publication.
In the months following publication, services that create indexes of journal contents, as well as services that index articles in various subjects, will include the new publication in their indexes (these indexes are frequently electronic databases of articles). These services provide access to other researchers to the information discovered by the original researcher, and will frequently touch off new areas of inquiry.
Between one and three years (usually) following publication, if the article is deemed important in its field, it may be included in a review article, which will synthesize recent, important research in a particular field to provide an overview of new discoveries, research trends, and the overall impact of current research.
Finally - usually several years after initial publication - information discovered during the initial research may be included in "common" or "accepted knowledge" sources like encyclopedias, dictionaries, and handbook.
This section is aimed at helping you understand, for example, what the difference between an encyclopedia and a handbook is. While most of the following categories of literature are present in every scholarly discipline, and may be intuitive to someone familiar with using reference works, the sciences also contain some specialized resources that don’t appear (or don’t appear frequently) in other disciplines.
Abstracts & Indexes: abstracting and indexing services are the primary means for a researcher to stay abreast of developments in his or her field, and to locate research that is relevant to his or her own area of inquiry. Some examples of major indexing and abstracting services are Chemical Abstracts or Mathematical Reviews. These services collect and publish citations and abstracts to scientific articles from journals around the world, which researchers can use to locate the full text of the relevant article. Abstracts are collected chronologically (for example, a single volume of one of these services would contain all of the articles published during a 6-month period). These services originated in print, but most can now be found in electronic formats.
Annual Review: published by Annual Reviews, a non-profit organization, these works provide critical reviews of the year’s most important scientific papers. These publications are considered extremely important in their disciplines (they are very highly cited, and each one is ranked among the top ten journals in its respective field). Usually, about 30 papers are reviewed each year. Annual reviews are published for nearly 40 different disciplines in four broad categories: biomedical, physical, social, and special sciences.
Bibliography (or Literature Guide): this kind of work is usually the product of extensive research into the scientific literature, aimed at providing the reader with the citations for a broad list of information sources of all types (books, journals, dictionaries, web sites, etc.) that can be found in the field.
Conference Proceeding: these consist of the papers that are presented at a professional conference, collected into a single volume. Conferences are one of the first areas that research findings are often discussed, and many of these papers will go on to be formally published in the scientific literature. Research results discussed in conferences may be preliminary.
Database (also sometimes called a Bibliographic Database): these sources represent a great leap forward in convenience for accessing scientific information, from citations and abstracts for articles across the sciences, to in some cases the full text of an article. Databases are electronic sources, but take care not to confuse them with sources from the “open Internet” or the “open Web.” While databases use Internet technology to transmit the information they contain, they are carefully maintained and designed, and the information they carry is evaluated by the database vendor to ensure accuracy and credibility.
Dictionary: works of this type provide short-to-moderate length definitions of terminology in a particular field. They are useful when researching, to define unfamiliar terms. The boundary between dictionary and encyclopedia blurs in the sciences at times; it is not unusual to see titles such as The Encyclopedic Dictionary of (Subject).
Directory: these works usually provide contact information for organizations, individuals, or institutions working in one of the scientific disciplines. For example, the International Research Centers Directory provides contact and other information for non-U.S. research organizations.
Encyclopedia: encyclopedias offer topic-specific entries that are usually aimed at a fairly general audience. Entries can be varied in length–more extensive or longer-lived topics tend to have very long entries, while newer or less important topics can have shorter entries. Some encyclopedias provide signed articles (that is, the author of each entry is indicated); some also provide bibliographic references for further research.
Handbook: handbooks typically collect useful quick-reference information, such as physical constants, handy formulas, chemical data, and diagrams in a single source to be used as a companion to the researcher’s work. While most handbooks are a single volume, some–like Beilstein’s Handbuch–consist of multiple volumes published over the years. However, their purpose remains to be sources for quick lookup of relevant data.
Journals: journals are the most important source for a researcher to consult, as they constitute the “moving front” of scientific research. As scientists perform experiments, collect data, and evaluate the results, their goal is usually to publish the information in a scholarly journal, where it can be distributed to the larger scientific community. To pass muster with the scientific community, journal articles undergo peer review, during which they are read by other experts in the field, who will comment on the article’s quality and importance, suggest areas for improvement, or ask additional questions about the research.
Journal articles vary somewhat from discipline to discipline, but typically contain the following elements:
Journal articles should contain sufficient information for another scientist, skilled in the field, to reproduce the experiment.
Letters Journal: the Letters Journal is a source for the publication of preliminary research findings. Physics was one of the first disciplines to use this mechanism to distribute research information, but it is common among many disciplines now. Note that this is an actual publication of results, although they may be considered “preliminary” results. Contrast this with the pre-print, which, while it has been accepted for publication, has not yet been published in a journal.
Pre-print: the “pre-print” is an accepted–but not yet published–scientific paper. It is usually distributed in order to get the findings into the field. Like the letters journal, this method of distribution originated in the field of physics, but has been adopted by other disciplines.
Website: this is a fairly self-explanatory “literature type,” but has a couple features that bear mentioning. First, the web has been a major boon to the distribution of scientific information–numerous sites have been created specifically to meet the needs of scientists in industry, academia, and other pursuits. However, with scientific websites like any site, one must be cautious to verify its accuracy, credibility, and currency before assuming the information is valid. See Finding and Evaluating Information on the World Wide Web for more information.