Help:Introduction to referencing/4
Now you know how to add sources to an article, but which sources should you use? Wikipedia articles require reliable, published sources that directly support the information presented in the article.
The word "source" in Wikipedia has three meanings: the work itself (for example, a document, article, paper, or book), the creator of the work (for example, the writer), and the publisher of the work (for example, Cambridge University Press). All three can affect reliability.
As a general rule of thumb, the more people engaged in checking facts, analyzing legal issues, and scrutinizing the writing of a publication, the more reliable it is. 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 host "blogs" on their websites. They may be reliable if they are written by the publisher's professional writers, but posts by readers should not be used as sources.)
Self-published media, where the author and publisher are the same, such as books, patents, newsletters, personal websites, open wikis, personal or group blogs and tweets, are largely not acceptable as sources. However, if an author is an established expert with a previous record of third-party publications on a topic, their self-published work may be considered reliable for that topic, though third-party publications are still preferable.
Whether a source is usable also depends on context. Bear in mind that sources that are reliable for some material are not reliable for other material. You should always try to find the best possible source for the information you have. For information about living people, only the most reliable sources should be used. On the other hand, self-published sources can sometimes be used as sources of information about themselves.
These are general guidelines, but the topic of reliable sources is a complicated one which it is impossible to fully cover here. You can find more information at Wikipedia:Verifiability and at Wikipedia:Identifying reliable sources.