Reliable sources typically include: articles from mainstream magazines or newspapers (particularly scholarly journals), or books by recognized authors (basically, books by respected publishers). Online versions of these are usually accepted, provided they're held to the same standards. User generated sources (like Wikipedia) are to be avoided. Self-published sources should be avoided except for information by and about the subject that is not self-serving (for example, citing a company's website to establish something like year of establishment).
Articles are to be written from a neutral point of view. Wikipedia is not concerned with facts or opinions, it just summarizes reliable sources. Real scholarship actually does not say what understanding of the world is "true," but only with what there is evidence for.
