Wikipedia:Verifiability
Responsibility for providing citations: All content must be verifiable. The burden to demonstrate verifiability lies with the editor who adds or restores material, and it is satisfied by providing an inline citation to a reliable source that directly supports the contribution.
Using inline citations, provide reliable, published sources for all:

direct quotations,
material whose verifiability has been challenged
material whose verifiability is likely to be challenged, and
contentious matter about living and recently deceased persons.
The cited source must clearly support the material as presented in the article. Cite the source clearly, ideally giving page number(s)—though sometimes a section, chapter, or other division may be appropriate instead; see Wikipedia:Citing sources for details of how to do this.
Any material lacking an inline citation to a reliable source that directly supports the material may be removed and should not be restored without an inline citation to a reliable source. Whether and how quickly material should be initially removed for not having an inline citation to a reliable source depends on the material and the overall state of the article. In some cases, editors may object if you remove material without giving them time to provide references. Consider adding a citation needed tag as an interim step. When tagging or removing material for lacking an inline citation, please state your concern that it may not be possible to find a published reliable source, and the material therefore may not be verifiable. If you think the material is verifiable, you are encouraged to provide an inline citation yourself before considering whether to remove or tag it.
Do not leave unsourced or poorly sourced material in an article if it might damage the reputation of living people or existing groups, and do not move it to the talk page. You should also be aware of how Wikipedia:Biographies of living persons also applies to groups.

Reliable sources: 

What counts as a reliable source: A cited source on Wikipedia is often a specific portion of text (such as a short article or a page in a book). But when editors discuss sources (for example, to debate their appropriateness or reliability) the word source has four related meanings:

The work itself (the article, book:  "That book looks like a useful source for this article.") and works like it ("An obituary can be a useful biographical source", "A recent source is better than an old one")
The creator of the work (the writer, journalist:  "What do we know about that source's reputation?") and people like them ("A medical researcher is a better source than a journalist for medical claims").
The publication (for example, the newspaper, journal, magazine:  "That source covers the arts.") and publications like them ("A newspaper is not a reliable source for medical claims").
The publisher of the work (for example, Cambridge University Press:  "That source publishes reference works.") and publishers like them ("An academic publisher is a good source of reference works").
All four can affect reliability.
Base articles on reliable, independent, published sources with a reputation for fact-checking and accuracy. Source material must have been published, the definition of which for the purposes of Wikipedia is made available to the public in some form. Unpublished materials are not considered reliable. Use sources that directly support the material presented in an article and are appropriate to the claims made. The appropriateness of any source depends on the context. Be especially careful when sourcing content related to living people or medicine.
If available, academic and peer-reviewed publications are usually the most reliable sources on topics such as history, medicine, and science.
Editors may also use material from reliable non-academic sources, particularly if it appears in respected mainstream publications. Other reliable sources include:

University-level textbooks
Books published by respected publishing houses
Mainstream (non-fringe) magazines, including specialty ones
Reputable newspapers
Editors may also use electronic media, subject to the same criteria (see details in Wikipedia:Identifying reliable sources and Wikipedia:Search engine test).

Best sources: The best sources have a professional structure for checking or analyzing facts, legal issues, evidence, and arguments. The greater the degree of scrutiny given to these issues, the more reliable the source.

Newspaper and magazine blogs: Some newspapers, magazines, and other news organizations host online pages, columns or rolling text they call blogs. These may be acceptable sources if the writers are professionals, but use them with caution because blogs may not be subject to the news organization's normal fact-checking process. If a news organization publishes an opinion piece in a blog, attribute the statement to the writer, e.g. "Jane Smith wrote ..." Never use the blog comments that are left by the readers as sources. For personal or group blogs that are not reliable sources, see § Self-published sources below.

Reliable sources noticeboard and guideline: To discuss the reliability of a specific source for a particular statement, consult Wikipedia:Reliable sources/Noticeboard, which seeks to apply this policy to particular cases. For a guideline discussing the reliability of particular types of sources, see Wikipedia:Reliable sources. In the case of inconsistency between this policy and the Wikipedia:Reliable sources guideline, or any other guideline related to sourcing, this policy has priority.

Sources that are usually not reliable: 

Questionable sources: Questionable sources are those that have a poor reputation for checking the facts, lack meaningful editorial oversight, or have an apparent conflict of interest.
Such sources include websites and publications expressing views widely considered by other sources to be promotional, extremist, or relying heavily on unsubstantiated gossip, rumor, or personal opinion. Questionable sources should be used only as sources for material on themselves, such as in articles about themselves; see below. They are not suitable sources for contentious claims about others.
Predatory open access journals are considered questionable due to the absence of quality control in the peer-review process.

Self-published sources: Anyone can create a personal web page, self-publish a book, or claim to be an expert. That is why self-published material such as books, patents, newsletters, personal websites, open wikis, personal or group blogs (as distinguished from newsblogs, above), content farms, Internet forum postings, and social media postings are largely not acceptable as sources. Self-published expert sources may be considered reliable when produced by an established subject-matter expert, whose work in the relevant field has previously been published by reliable, independent publications. Exercise caution when using such sources: if the information in question is suitable for inclusion, someone else will probably have published it in independent, reliable sources. Never use self-published sources as third-party sources about living people, even if the author is an expert, well-known professional researcher, or writer.

Self-published or questionable sources as sources on themselves: Self-published and questionable sources may be used as sources of information about themselves, usually in articles about themselves or their activities, without the self-published source requirement that they are established experts in the field, so long as:

The material is neither unduly self-serving nor an exceptional claim;
It does not involve claims about third parties;
It does not involve claims about events not directly related to the source;
There is no reasonable doubt as to its authenticity; and
The article is not based primarily on such sources.
This policy also applies to material made public by the source on social networking websites such as Twitter, Tumblr, LinkedIn, Reddit, and Facebook.

Wikipedia and sources that mirror or use it: Do not use articles from Wikipedia (whether English Wikipedia or Wikipedias in other languages) as sources, since Wikipedia is a user-generated source. Also, do not use websites mirroring Wikipedia content or publications relying on material from Wikipedia as sources. Content from a Wikipedia article is not considered reliable unless it is backed up by citing reliable sources. Confirm that these sources support the content, then use them directly.
An exception is allowed when Wikipedia itself is being discussed in the article. These may cite an article, guideline, discussion, statistic, or other content from Wikipedia (or a sister project) to support a statement about Wikipedia. Wikipedia or the sister project is a primary source in this case and may be used following the policy for primary sources. Any such use should avoid original research, undue emphasis on Wikipedia's role or views, and inappropriate self-reference. The article text should clarify how the material is sourced from Wikipedia to inform the reader about the potential bias.

Accessibility: 

Access to sources: Do not reject reliable sources just because they are difficult or costly to access. Some reliable sources are not easily accessible. For example, an online source may require payment, and a print-only source may be available only through libraries. Rare historical sources may even be available only in special museum collections and archives. If you have trouble accessing a source, others may be able to do so on your behalf (see WikiProject Resource Exchange).

Non-English sources: 

Citing: Citations to non-English reliable sources are allowed on the English Wikipedia. However, because this project is in English, English-language sources are preferred over non-English ones when they are available and of equal quality and relevance. As with sources in English, if a dispute arises involving a citation to a non-English source, editors may request a quotation of relevant portions of the original source be provided, either in text, in a footnote, or on the article talk page. (See Template:Request quotation.)

Quoting: If you quote a non-English reliable source (whether in the main text or in a footnote), a translation into English should accompany the quote. Translations published by reliable sources are preferred over translations by Wikipedians, but translations by Wikipedians are preferred over machine translations.