Cascading Style Sheets (CSS) is a technology used to define the presentation of webpages.
Stylesheets are designed to separate the presentation of a webpage from the content.
In theory, this allows for the presentation of a site to be changed easily without concern for the content. For example, a site could be redesigned or optimized for different devices (such as mobile phones) using CSS. In reality, the technology is not yet sufficiently mature for this in the majority of real-world situations.
Cascading Style Sheets – Wikipedia