From Joomla! Documentation
A Cascading Style Sheet or CSS is used to control the presentation of an XHTML page. For example, a CSS file will often control the font, margins, color, background graphics, and other aspects of a web page's appearance. CSS allows you to separate the content of an XHTML page from it's appearance. In Joomla!, CSS files (for example, template.css) are normally part of the template.
It was conceived to ease the separation of content (written in XML markup) and presentation (written in CSS) of documents. CSS can be used to define and alter almost all presentation aspects of a document, like colours, font sizes and types, text direction, element sizes and positioning, etc.
CSS can also be used to improve accessibility of documents. It also provides mechanisms to present the same XML markup in different ways, optimised for different devices like screen-readers, speech-synthesizing browsers, printers, and Braille-based, tactile devices.
Core Joomla! CSS
Joomla provides many class- and id-attributes in its HTML output, which can be used to alter its presentation through the use of CSS definitions for said classes and ids.