What are the three primary parts of a CSS definition?

The three primary parts of a CSS definition are:

selector{property:value;}

The selector can be one of three things or a combination of the three. These are element (such as <div> or <img />), id (#), and class (.).

The property can be a number of rules ('background', color, font-weight). A good place to reference these is w3schools.org.

The value is specific to the property. For example, if the selector is 'color', value could be 'red' or '#ccc', but it couldn't be 'bold'.