CSS, or Cascading Style Sheets, is a style sheet language used to describe a document's look and formatting in a markup language. It is most commonly used to style web pages written in HTML and XHTML but can also be applied to any XML document, including SVG and XUL.

For example, the following CSS code defines a style for a heading element, specifying that it should be displayed in a bold, 24-point font:

h1 {
  font-weight: bold;
  font-size: 24pt;
}

To apply this style to a heading element on a web page, we simply need to include the appropriate HTML tag and specify the class or id of the style we want to use, like so:

<h1 class="heading">My Heading</h1>

We can use a class selector to apply the above style to any element with a class of heading, like so:

.heading {
  font-weight: bold;
  font-size: 24pt;
}