I think the thing you are asking about is technically called a "css selector". A css selector seems to tie a css rule and its style declarations to the HTML elements you are trying to style. You can learn more by googling for "css selector" and at some of the links below:
http://www.w3schools.com/css/css_syntax.asp
https://developer.mozilla.org/en-US/...ed%2FSelectors
http://www.w3schools.com/cssref/css_selectors.asp