In categorizing sources, we have talked about the audience to which a source is directed. But there are other perspectives to consider as well.
One possible perspective is the method used to publish a source. How and by whom is the source made accessible to the public? When you start searching for literature you'll come across sources that are 'traditionally' published.
Traditionally published sources come from a commercial or academic publisher (e.g., a university press).
Books/e-books, focusing on different audiences:
Periodicals (publications that are published regularly: daily, weekly, monthly, or quarterly), again focusing on different audiences:
Publishers manage the editing, layout, printing, advertising, and distribution of traditional sources such as books, journals, magazines, and newspapers. They are widely available from public libraries, university libraries, (online) bookstores, but also from supermarkets and gas stations.
Newspapers, professional/trade journals and many magazines are available online as well as in print form. Nearly all scholarly journals have established digital versions (called e-journals). Any new quality journal will almost always be online-only these days, for economic reasons. Print books are also increasingly available as e-books.