When we call a journal "popular", that doesn't mean it's a top-seller -- but it does mean that the content is written to inform and entertain the general public (anyone interested in their topics).
Academic journals are considered scholarly sources because their articles intend to further knowledge, by presenting the results of peer-reviewed research or proposing new theories. Their intent is to educate students and scholars.
Professional journal articles may also present research results, have discussions on theory; they also cover news pertaining to their profession. But their aims are to inform practitioners, and to further best practices and standards.
For example, on the topic of organic gardening, these magazines or journals would be:
(Table from Rutgers University, Popular Literature vs. Scholarly Peer-Reviewed Literature: What's the Difference? . The rest of that webpage has other very useful tips, too.)
Newspapers share many of the characteristics as popular magazine articles, but they can focused on:
They're good resources for information on the people, events, and places that matter to a specific region. However, in locally-focused newspapers, coverage of national and international news often comes from a newswire (for example, AP/Associated Press), so you will find essentially the same story in more than one newspaper. Keep in mind that newspapers will reflect the biases of their communities, so if you are researching topics of international import, look for newspapers from other countries besides your own.
Government or Agency Reports focus on specific issues, and sometimes contain detailed research reports as well as information about policies and programs that aim to fix problems associated with the topic. Look for documents produced by agencies that have responsibility in this area (for example, U.S. Department of the Interior, Environmental Protection Agency; United Nations Environment) or reports for advocacy organizations (for example, Union of Concerned Scientists). Keep in mind, though, that all of these groups have some kind of political agenda and bias; that's not a reason to reject their information, but it's wise to learn more about their advocacy or opinions so you can factor that information into your essay too.
What about Books? Not that we have anything against books, but often these contain more material than you would use for an essay, especially if your instructor has asked you to include multiple sources of information. (If you are further interested in the topic, definitely check out the books on Library shelves and read from our e-book collections!) There are two exceptions to this general rule, though; you may find useful material for your essays in:
Pick the publication source(s) that best meet your information need. Do you want: general overview vs. focused research? Or written for the professional vs. written for the general public? Or up-to-the-minute info vs. well-established fact or historical background? Search in the publication type(s) most likely to present what you're seeking.
Also, in order to properly cite a source, you need to identify its publication type to determine how to correctly format that citation. Identifying the publication type can be more challenging for online sources since there is often less context - ask a librarian for help if you're unsure.
Like members of any profession, scientists have their own jargon. Elaborate terms are shortened to abbreviations to make them easier to say and write (for example, DNA instead of deoxyribonucleic acid).
They also have different meanings for words than common usage. (For example, in the phrase "theory of evolution", the word "theory" is used as part of the scientific method, meaning "the best explanation we have" for how organisms evolve. That explanation is accepted by scientists as truth - but people who believe in intelligent design interpret the word "theory" to mean that there is doubt about evolution.)
Jargon can make scholarly, academic or professional articles difficult to understand. If you find yourself confused, ask a librarian, who can point you to dictionaries and other resources to explain scientific jargon.