In categorizing sources, we have so far looked at the audience to which a source is directed. But there are other angles as well. One of them is 'publication status -- how a source is published. Some of the sources you'll come across will be 'formally' published, others will not.
Formally published sources (often referred to as 'mainstream publications') come from a commercial or scholarly publisher. This type of source is widely available, for instance from (online) bookstores and libraries, but also from supermarkets and newsstands.
Books/e-books, which focus on different audiences.
Periodicals, which are published regularly (daily, weekly, monthly, or quarterly).
More and more periodicals are available in both print and digital format. For instance, nearly all scholarly journals have established digital versions (called e-journals). Some of them, particularly newer journals, are now published in digital form only. Books are also increasingly available as e-books.
Relatively new are podcasts, which are mainly published by newspapers. The New York Times's The Daily has become the genre's big hit. It publishes a 20-minute bulletin every weekday.
What about the quality of formally published sources?
Among the vast amount of formally published sources, scholarly journals and books are by far the most authoritative. That has everything to do with the quality-control standards in scholarly publishing, which gives these publications maximum quality and credibility.