Meryn Stol added this answer. Meryn Stol You see two things: 1. A CSS selector: ul#menu li ul.sub-menu 2. An opening (curly) brace { , which should be followed by zero or more css declarations, and a closing brace } . The CSS selector determines to which elements the CSS declarations ("styles" from now on) are applied to. The easiest way to understand the CSS selector is to read its individual components from right to left. To start with, the styles only apply to ul elements with class "sub-menu". I.e. you'd see something like <ul class="sub-menu"> in the HTML. Furthermore, it only applies to those ul elements which appear somewhere nested inside a li element. Those li elements in turn need to be nested inside an ul element with id "menu", i.e. something like <ul id="menu"> An example of what is matched: 1 2 3 4 5 <ul id="menu"> <li> <ul class="sub-menu">THIS IS MATCHED</ul> </li> </ul> The selector in fact matches many more types of element hierarchies, because both <li> and <ul... - Meryn Stol