Before we get too far into specific rules, it probably makes sense to discuss the general types of "rules" we'll discuss. There are three categories, descending order of importance:
Policies — Policies are about standards for acceptable behavior and content that are more or less mandatory for editors to follow. Violate the wrong policy and you could be blocked from editing.
Guidelines — Guidelines tend to describe how policies should be implemented in practice. They're not as "mandatory" as policies, but you should follow them.
Essays — Essays are unofficial rules, explaining certain attitudes of the editorial community that may or may not be shared by most editors.
For each rule ahead, we'll be sure to tell you what type of rule it is, so you know how it fits into the overall ruleset.