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Wikipedia articles require reliable, published sources that directly support the information presented in the article. Now you know how to add sources to an article, but which sources should you use?
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, 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.)
Self-published media, where the author and publisher are the same, including newsletters, personal websites, books, patents, open wikis, personal or group blogs, and tweets, are usually not acceptable as sources. The general exception is where the author is an established expert with a previous record of third-party publications on a topic; in this case, their self-published work may be considered reliable for that topic (but not other topics). Even then, third-party publications are still preferable.
Whether a source is usable also depends on context. 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 written by articles' subjects can sometimes be used as sources of information about themselves.
These are general guidelines, but the topic of reliable sources is a complicated one, and is impossible to fully cover here. You can find more information at Wikipedia:Verifiability and at Wikipedia:Reliable sources. There is also a list of commonly used sources with information on their reliability.