Yes, CSS rules!, but what is a css rule?
A css rule defines how an alement (or a list of them). It has the selector, which indicates which element or elements are targetted, the property, which defines what are we changing or specifying in the element and then the value for the property. The group between a property and a value is a declaration. Every declaration must end with a semi colon, and all declarations for a selector must be wrapped in curly braces.
The group of declaration and the selector is a rule.