CSS Units is a guide to understanding and using the different units of measurement available in Cascading Style Sheets (CSS). This guide covers the basics of CSS units, such as the different types of units available, how to use them in your code, and how to convert between them.
From Wikipedia
Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language, such as HTML or XML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.