Here are some places where you can find out more about CSS Syntax, Selectors, Properties and Values.
Mozilla Developers Network: MDN
This site sets the standard as a reference for all things web.
This is the main area for all things CSS at MDN
This is an excellent resource not only on CSS but also for front-end web development in general.
A ruleset is what applies CSS to an element or elements. This is the core syntax for CSS.