As explained in the section on Documents, the periodical evolved from the book, and the reason is that the periodical filled two main needs that the printed book could not:

1. Publication of current information. Periodical publishers and book publishers operate under very different business models, and the former are able to publish new information far more quickly and inexpensively than the latter.
2. Publication of information that does not lend itself to publication in book format, either because even a full treatment of the subject would be too short to warrant publication as a book, or because the audience for the information is so specialized that publication as a book would represent too great a market risk for a publisher to assume.¹

There are dozens of types of periodicals. Four important types are described below:

• Scholarly. Often called journals. Intended for academic audiences. Many scholarly periodicals are published by university presses, learned societies, and other not-for-profit publishers, but others are published by commercial (or "for-profit") publishers like Routledge, Brill, De Gruyter, Springer, Brepols and, Sage. Not all scholarly periodicals are peer reviewed.
• Popular. Often called magazines. Almost always published by commercial ("for profit") publishers, and often cease publication when they become unprofitable for the publisher. Intended for popular audiences, but can cater to smaller audiences (like hobbyist magazines) as long as the audience is segmented such that the magazine can still be profitable. Pornographic magazines, like the gay fetish magazine Bound and Gagged, are also technically commercial, and form part of the Library's periodical collection.
• Trade. Often called trade journals or trade newspapers. Intended for members of a profession (Chronicle of Higher Education), occupation (Railway Carmen's Journal), or industry (Hollywood Reporter). They often resemble newspapers in frequency of publication (weekly and even daily) and appearance (printed on large format, inexpensive paper, with no cover).
• Alternative. Have an acknowledged political bias that is considered to be outside the "mainstream".  Alternative periodicals are usually not expected to turn a profit.They are intended to motivate readers to action, or form coalitions from like-minded people. They are often low-budget newsletters, but can also be expensively produced magazines or organs of societies and special interest groups. Alternative newspapers and periodicals are sometimes mistakenly referred to as "underground newspapers." Strictly speaking, an underground publication is one that is published secretly--in other words, the place of publication and identity of the publisher are not disclosed. Most alternative newspapers and periodicals in library collections do not meet this criterion.

These distinctions are simply a method for classifying sources; and, like all classification schemes, it provides a method for quickly completing a task, in this case the task is drawing certain conclusions about the nature of a source. The conclusions you draw should not be your final judgment on the question of the source's value. Classification schemes often obscure as much as they reveal about whatever they are attempting to describe.

Neither source type ("scholarly" or "popular") definitively indicates the value or reliability of a source, but recognizing the difference can sometimes make it easier to predict the probability of a source's value and reliability. You still need to evaluate each source critically.

There are many article databases for finding periodical articles.  These databases are often called article indexes, but they are essentially searchable bibliographies of journal articles organized by subject.

Because the Library does not subscribe to every journal, and because not all journals are digitized, and because not all digitized journals are available in a single collection, the article indexes provide the only efficient means of identifying relevant articles from across the widest possible range of periodical publications.

Most of these article indexes include a mixture of academic and popular sources (and remember that sometimes the distinction is not clear).

The two most important databases for identifying journal articles in history are America: History and Life, and Historical Abstracts. The two databases complement each other, and you will choose one or the other (or both) depending on the region you are researching:

Although America: History and Life, and Historical Abstracts are the two most "important" article indexes for historical research, there are several other very important article indexes, any of which might be crucial for research depending on the focus of your research:

For many more article indexes, see the "Bibliographies, Catalogs, and Indexes" section in our Guide to Online Reference Sources:

The collections below primarily comprise scholarly journals:

1. D.E. Davinson, The Periodicals Collection: Its Purpose and Uses in Libraries (London: Andre Deutsch, 1969), 38.