CSS is structured in a way that allows developers to separate the content and structure of a web page from its presentation and style. The structure of CSS includes selectors, properties, and values.

Selectors are used to identify the HTML elements that you want to style. They can be based on the element type (such as `p` for paragraphs or `h1` for headings), classes, IDs, or other attributes. For example, the following selector targets all paragraphs in the HTML document:

p {
  /* CSS rules here */
}

Properties are used to set the style of the HTML element. There are many different properties that you can use, such as `font-size`, `color`, `background-color`, and `border`. Each property has a value that determines how it will be applied to the element. For example, the following property sets the font size of a paragraph to 18 pixels:

font-size: 18px;

Values can be assigned to properties and are used to determine how the property is applied to the HTML element. For example, a value of bold could be assigned to the `font-weight` property to make text appear in `bold`. Different properties can have different values, and you can also use keywords such as `inherit` or `initial` to set specific values.

CSS is typically written in a separate file from the HTML document, and the two are linked together using a `link` tag in the HTML code. For example:

<link rel="stylesheet" type="text/css" href="styles.css">