Wikipedia:But it's true!
"But it's true!" is not a sufficient reason to keep information on Wikipedia. The purpose of Wikipedia is to be an encyclopedia of high quality. To pursue that mission, a number of standards, guidelines and policies have been set in place. Based on these policies, some information or articles will be deleted even though they may be true. For example, you may know something to be true because you have witnessed it. However, on Wikipedia, article content must be verifiable in reliable sources. Another example is an article created entirely to promote a certain product. Even if all the claims in the article are true, promotional articles are not permitted. As well, information about the details of a celebrity's private life may be deleted–even if it is demonstrably true and confirmed by reliable sources–on the grounds that Wikipedia is not an indiscriminate collection of all information; the reason there is an article about singer XYZ is because of their achievements as a singer, not because tabloids obsessively reveal all the lurid details about their dating life.
Reversion is a normal result
When submitting new articles to Wikipedia, or adding content to existing ones, you may be surprised if some or all of that content is reverted by another editor, or, in the case of new articles, the article is deleted. While newer editors are the ones most commonly surprised by this, even experienced editors can be caught off-guard when contributions are removed. This is especially true when the editor has taken great care to ensure the content provided was true. This essay is intended to help you understand why this might happen, and how to best handle the situation when it does.
Reversion is a normal part of the Wikipedia editing process, and should not be taken personally. The overwhelming majority of reverts are done in good faith. It is also likely that even when the other party disagrees with your edit, they believe you were acting in good faith as well. To understand why an edit was reverted, start by reading the edit summary on the revert, if one was provided. The summary might provide important clues as to the reason, perhaps by citing one or more policies or guidelines, or by referring you to the article's talk page.
Core content policies
Accuracy is important to any encyclopedia. For this reason, Wikipedia has developed three core content policies. Each of these policies functions much like one leg of a three-legged stool or table. If you leave off any one of the legs, it will fall. Likewise, any edits that do not meet all three of these policies are likely to be challenged or removed. It is not enough to meet just one or two of them.
Neutral point of view (or WP:NPOV)
All encyclopedic content on Wikipedia must be written from a neutral point of view, without bias, representing significant views fairly, accurately, and in due proportion to the prominence of each viewpoint. Wikipedia aims to describe disputes as represented in reliable sources, but not to engage in such disputes.
Editors naturally have their own points of view, but should strive in good faith to provide complete information and not to promote one viewpoint over another. Use non-judgmental language, neither sympathizing with or disparaging subjects. Where controversies exist, accurately indicate the relative prominence of each opposing view.
Verifiability (or WP:V)
One way we promote accuracy on Wikipedia is to provide a means for readers (and other editors) to independently verify the accuracy of our content. We do this by citing reliable sources. Contentious material that is unsourced or poorly sourced should be removed, even if the material appears on a page other than that person's biography. Personal knowledge, anecdotal experience or hearsay are not acceptable unless they can be somehow be verified with a reliable source. We don't necessarily doubt that what you are writing is true, but if we cannot verify it in a reliable source, then it does not belong in the article.
If an edit was removed because it did not cite a source (or did not cite a reliable source), try to find one. If your source is considered unreliable and you're not sure why, try asking politely for clarification on the article's talk page. Most editors will be happy to help. As a rule of thumb, user-submitted content on websites (IMDB, blog comments, etc.) will not qualify as reliable sources. Some examples of reliable sources are books published by reputable publishing houses and major newspapers and magazines.
No original research (or WP:NOR)
Like any encyclopedia, Wikipedia is not a publisher of original research or original, unpublished thought. Rather, its purpose is to collect and provide an overview of knowledge that has been published in other reliable sources.
Perhaps you know something is true, because you were there when it happened. That's great, but unless it's been written about in a reliable source, it's an example of original research, which is defined in part as "material for which no reliable source can be found." To show that your edit is not original research, you must be able to cite a reliable source that contains the same information. If you can't do that, then your edit constitutes original research, and is not appropriate for inclusion in an encyclopedia.
You may be even more convinced that you know something is true because you have a PhD in the subject matter and you are a recognized expert in the field. However, even though you are an expert in astronomy, you cannot create a Wikipedia article about a new celestial body you have seen through your telescope until this new planet is written about in reliable, published sources.
A more subtle form of original research is synthesis, which is drawing a conclusion by combining facts from multiple sources. As an example, suppose you have one source that says all ratchets are widgets, and another source which says all widgets are gadgets. It might seem obvious from this that all ratchets are gadgets, but such a statement would be synthesis. To include a statement that all ratchets are gadgets, you must find a reliable source that draws the same conclusion. A common form of synthesis is to take a reliably sourced fact A and reliably sourced fact B, and then go on to do your own unsourced conclusion C.