Informally published sources are produced by organizations whose primary activity is NOT publishing. In other words, these sources are not tied to or controlled by established publishers, who seek profit on the research or information produced.
Informally published sources typically come from:
Informally published information includes, but is not limited to, the following sorts of materials:
Informally published sources may be available in both print and digital format (e.g. government reports), but these documents are in many cases published only online.
Compared to formally published sources, quality control of informally published sources is less rigorous or sometimes even non-existent. As a result, some of these sources are unreliable.
That said, many of them are very reliable and useful. For instance, very recent information or research may not yet have been formally published in a journal article or book - but might be available in a working paper or conference paper. Reports from professional organizations may examine particular topics in more depth, or on a more practical level, than mainstream publications. Government reports, scientific or not, are reliable. Many prominent academics, journalists, and public figures have blogs that are considered trustworthy.
Wikipedia can serve as a convenient starting point for finding information on a new topic. Watch this short video that explains the advantages and disadvantages of Wikipedia, and how you can best make use of it [sound not required].