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 have a notice at the top of their pages, and the prefix "Wikipedia:" or "WP:" before their 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'. Knowing these basics makes discussions and editing easier and more productive.