CSS defines the look-and-feel of HTML elements. This is called styling.
A style is written as a rule-set which consists of a selector and a declaration block.
A selector defines which HTML elements are selected.
A declaration block lists properties, such as color or border, and their values.
Here's a visual overview of a rule-set.
Explanation
A selector references the HTML elements to which the style applies.
A declaration block consists of one or more declarations separated by semicolons.
A property is the characteristic that will be styled (e.g. color, border).
The value is the value for the property (e.g. color property can have blue as value).
Declarations end with a semicolon, and declaration blocks are surrounded by curly braces.
This entire description represents a single rule-set.