The fundamental principles of Wikipedia may be summarized in five pillars:
Wikipedia is free content that anyone can use, edit, and distribute: Since all editors freely license their work to the public, no editor owns an article and any contributions can and will be mercilessly edited and redistributed. Respect copyright laws, and never plagiarize from sources. Borrowing non-free media is sometimes allowed as fair use, but strive to find free alternatives first.
Editors should treat each other with respect and civility: Respect your fellow Wikipedians, even when you disagree. Apply Wikipedia etiquette, and don't engage in personal attacks. Seek consensus, avoid edit wars, and never disrupt Wikipedia to illustrate a point. Act in good faith, and assume good faith on the part of others. Be open and welcoming to newcomers. Should conflicts arise, discuss them calmly on the appropriate talk pages, follow dispute resolution procedures, and consider that there are many articles on the English Wikipedia to improve and discuss.
Wikipedia has no firm rules: Wikipedia has policies and guidelines, but they are not carved in stone; their content and interpretation can evolve over time. The principles and spirit matter more than literal wording, and sometimes improving Wikipedia requires making exceptions. Be bold but not reckless in updating articles. And do not agonize over making mistakes: every past version of a page is saved, so mistakes can be easily corrected.