CSS rule

CSS rule is a statement that tells browsers how to render particular elements on an HTML page. It is a single section of CSS including the selector, the curly braces, and the different lines with properties and values. For example:

p { 
	color: #000; font-size: 20px;
}

CSS rule consists of a selector followed by a declaration block:

Selector

Selector points to the HTML element you want to style. It consists of everything up to the first left curly bracket.

Declaration block

Declaration block is the container that consists of one or more declarations separated by semicolons. Declaration groups are surrounded by curly braces. Each declaration block includes a property name and a value, separated by a colon:

Property

Property is an aspect of that element that you are choosing to style. There can only be one property within each declaration.

Value

Value is the exact style you wish to set for the property.