Style sheets in CSS are made up of rules.

body { color: purple;
       background-color: #d8da3d } 

Each rule has three parts:

the Selector (in the example: “body”)

tells the browser which part of the document is affected by the rule;

the Property ('color' and 'background-color' are both properties)

specifies what aspect of the layout is being set;

the Value ('purple' and '#d8da3d')

gives the value for the style property.

The Property and Value parts are called the Declaration part

Source : http://www.w3.org/Style/Examples/011/firstcss