Why references are important
How to add them
Citations the easy way
Review of what you've learned
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.
Reliable sources are those with a reputation for fact-checking and accuracy. They tend to have an editorial process with multiple people scrutinizing work before it is published. 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 news coverage (not opinions) from mainstream newspapers.
Self-published media, where the author and publisher are the same, are usually not acceptable as sources. These can include newsletters, personal websites, press releases, patents, open wikis, personal or group blogs, and tweets. 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 particular topic.
Whether a source is usable also depends on context. Sources that are reliable for some material are not reliable for other material. For instance, otherwise unreliable self-published sources are usually acceptable to support uncontroversial information about the source's author. You should always try to use the best possible source, particularly when writing about living people.
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.