"It ain't ignorance causes so much trouble; it's folks knowing so much that ain't so."
This is a non-exhaustive list of sources whose reliability and use on Wikipedia are frequently discussed. This list summarizes prior consensus and consolidates links to the most in-depth and recent discussions from the reliable sources noticeboard and elsewhere on Wikipedia.
Context matters tremendously, and some sources may or may not be suitable for certain uses depending on the situation. When in doubt, defer to the linked discussions for more detailed information on a particular source and its use. Consensus can change, and if more recent discussions considering new evidence or arguments reach a different consensus, this list should be updated to reflect those changes.
Reliability is an inquiry that takes place pursuant to the verifiability policy and the reliable sources guideline. Note that verifiability is only one of Wikipedia's core content policies, which also include neutral point of view and no original research. These policies work together to determine whether information from reliable sources should be included or excluded.
As a general rule, more reliable sources have more people engaged in checking facts, analyzing legal issues, and scrutinizing the writing in a publication. Academic and peer-reviewed publications are usually the most reliable sources. Other reliable sources include university textbooks, books published by respected publishing houses, magazines, journals, and mainstream newspapers. (Be aware that some news organisations and magazines, such as CNN's iReport, host "blogs" and user-written articles on their websites. These may be reliable if they are written by the publisher's professional writers, but posts by readers are not usually considered reliable sources.)
Why references are important? References are vital for the quality and usefulness of Wikipedia articles. If you don't believe me, don't take my word for it; At the Wikimania conference 2006, Jimmy Wales himself said he believed Wikipedia should focus more on the accuracy of our existing material instead of creating new material. Since then, we have broken the 5 million article mark and there is still much room for improvement.
We tell people to not rely on Wikipedia as their sole source of information but rather use it as a starting point for further research. For this to work, articles not only need external links to guide readers to further information, but also a list of sources we used to write the Wikipedia article so the reader can go and check our material against that of the sources to check our accuracy. (Source)
Editorial commentary, analysis and opinion pieces, whether written by the editors of the publication (editorials) or outside authors (op-eds) are reliable primary sources for statements attributed to that editor or author, but are rarely reliable for statements of fact. Human interest reporting is generally not as reliable as news reporting, and may not be subject to the same rigorous standards of fact-checking and accuracy (see junk food news).