Information sources are often classified as physical (print, analog) versus online (electronic, digital) text versus audio/video and book versus journal.

Here are some common information source types with descriptions of how current their information usually is, what kind of information is contained in them, and where to find them.

Journal Articles

• Currency: Current within a few months to a few years of publication. Look at the list of references used. What is the most recent date you can find? That should tell you when they stopped researching and started writing. But bear in mind that experimental/observational data they gathered may be a year or two older than that.
• Type of Information: Most recent research within the subject of the journal. Scholarly journal articles are important in all academic subject areas, but especially in the sciences, where most researchers do not write books.
• Where to Find: Print journals are delivered to subscribers and libraries. Some journals are Open Access and make all their content online for free. Some journals allow authors to keep a copy of their articles online in a repository and you can usually find these through Google Scholar. Libraries subscribe to article databases. Those subscriptions make millions of articles available to users at those institutions.

Magazine Articles

• Currency: News magazine articles should be current within a few days to a few months of publication. But many magazine articles are based on scholarly articles, so their information is not as new.
• Type of Information: Current events and editorials (news magazines). Non-scholarly articles about topics of interest within the subject of the magazine.
• Where to Find: Print magazines are delivered to homes and libraries. Some magazines have an online presence, but access to older articles may require a subscription. Some library databases have full-text articles from magazines.

Newspaper Articles

• Currency: Current within a few minutes to a day of publication. Corrections made after the fact can change content later.
• Type of Information: Current events and editorials.
• Where to Find: Print newspapers are delivered to homes and libraries. Many newspapers have an online presence but access to older articles may require a subscription. Libraries can subscribe to newspaper databases.

Monographs (a.k.a. Scholarly Books)

• Currency: Information may be two or three years old. Just like with journals, look for the most recent date in the bibliography, and that should tell you around when the author(s) were researching and writing. Bear in mind that experimental/observational data the author(s) gathered may be a year or two older than that.
• Type of Information: Scholarly research on a topic. Not as recent as a journal article, but may address a whole subject rather than just a piece of it. Monographs are very important in the humanities.
• Where to Find: Monographs are primarily available through academic libraries. Some are in print, some are e-books. These e-books are not available to consumers but are generally meant to be read via a web browser or downloaded as a PDF. 

Nonfiction Books

• Currency: Varies widely. Books on hot topics may be published within a few weeks but, as a result, they may contain errors. Other books take two or more years to get to print, and the research may be even older.
• Type of Information: Non-scholarly information and opinion. 
• Where to Find: Nonfiction books are found in bookstores and mainly public libraries. e-book versions may be available for consumers via Kindle, Nook, iPad, etc. Nonfiction books on academically relevant topics that are of reasonably high quality are also collected by academic libraries. Some of them may be available through academic libraries in formats that are read in web browsers or downloaded as PDF.

Reference Resources

• Currency: Print reference resources often have annual updates, so the information in them should be only about a year old. Online reference resources may be updated continuously. Many statistical resources have older data because it takes a long time to organize it all. Historical resources summarize and synthesize established knowledge, rather than keeping up with the newest findings. The important thing is that reference resources usually tell you how old their information is. 
• Type of Information: Summary and synthesis of what is known about a topic. Materials to be referred to; for example, facts and figures, dates, names, measurements, statistics, quotations, instructions, equations, formulae, definitions, explanations, charts, graphs, diagrams, maps. 
• Where to Find: Traditionally, reference resources are available as books or series of books. They can be purchased by consumers but are often far too expensive. They can be found in the reference sections of public and academic libraries. Not all print reference sources are books; for example, there are also maps. More and more reference resources are available in online format, and as they go online, they become less and less linear, taking advantage of the ability to link and include multimedia. Online reference resources are available through specialized library databases, and there are also many of them on the web. Some are free and some require an individual subscription.

Textbooks

• Currency: Varies widely. Some textbook editors publish a new edition every year, and their information should be current within a year or two of the edition's publication date. Other textbooks in less time-sensitive disciplines may contain information that is more historical in nature.
• Type of Information: Information on a topic arranged in such a way that a beginner can acquire knowledge about that topic systematically. Textbooks are meant to be used as part of taking a course but are usually written so that they are complete and understandable on their own. Textbooks may have supplemental materials like questions to guide your reading or self-quizzes as well as accompanying multimedia material. Some e-textbooks come with fully integrated multimedia. 
• Where to Find: In libraries that have a physical location and physical collection, some textbooks may be in the stacks or held on reserve for short-term loan. Most textbook publishers do not make textbooks available as e-books for libraries. Instead, textbooks, whether print or e-book, must be purchased by the individual student. Some textbooks may also be rented for the duration of the course.

Gray Literature

Gray literature is a huge category that encompasses a wide variety of documents that have not been published in the traditional sense. Gray literature includes:

• Unpublished conference papers
• Unpublished theses and dissertations
• Presentations
• Working papers
• Notes and logs kept by researchers
• Academic courseware, professors' teaching notes, students' lecture notes
• Company annual reports
• Project and study reports
• Institutional reports
• Technical reports
• Reports put out by government agencies
• Data and statistics
• Unpublished letters and manuscripts
• Patents, technical standards
• Newsletters, product catalogs, and certain other types of ephemera with a strong informational value
• Preprints of articles
• And much more!

Certain kinds of gray literature can be found in databases. Others are best found by searching the web. 

Sometimes it is difficult to ascertain the author or organization responsible for the information, which can make gray literature may be difficult to cite.