A Joomla! template is a set of files that contain the HTML and CSS code defining what your website will look like. HTML is the code used to build web pages and CSS is the code used to style them. You could say that HTML is the bricks and mortar of your site, whereas CSS provides the wallpaper and paint. Let's see how this works.
Without any CSS applied, your web browser would display a typical Joomla! site as follows:
With CSS styles applied, the same content is displayed—but now the overall page layout and design is added:
Actually, a template contains more than pure HTML and CSS. To the basic HTML, it adds some instructions (written in the PHP language) that tell Joomla! what content it should place at which position within the HTML page structure. This way, the template instructs Joomla! exactly where it should put the central content, menus, and other modules.
So that's what a template does. Joomla! as such generates only the basic HTML; the template adds to...