This page in a nutshell: On Wikipedia, policies define the fundamental principles, while guidelines either advise how to apply these policies or provide general consistency.
The primary goal of Wikipedia is to provide a high-quality encyclopedia for free. Although Wikipedia does not have many strict rules, it operates on five fundamental principles. Wikipedia's policies and guidelines are developed by the community to clarify these principles and describe the best way to apply them, resolve conflicts, and otherwise further our goal of creating a free and reliable encyclopedia.
"Wikipedia:" or "WP:" will help indicate what is an official policy/guideline.
Essentially, policies express the fundamental principles of Wikipedia in more detail and guidelines advise how to apply policies and provide general consistency across articles. Formal policies and guidelines can be identified by their notice placed at the top of their pages, and the prefixes "Wikipedia:" or "WP:" before the page name.
While there is usually a policy or guide for every issue imaginable, no one is expected to know all of them! Luckily there are a handful upon which all others are based. The next few pages describe the most significant of these, representing the general spirit of Wikipedia's 'rules'. If followed, discussions and editing will be easier and more productive.