A template is used to manipulate the way content is delivered to a web browser or screen reader. Here are some key examples of how a template is used to manipulate your site's content.
The template is the place where the design of the main layout is set for your site. This includes both where you place different elements (components, modules, and plugins), some examples of which are as follows.
If the template is designed for provide choices, you can also "dynamically" alter the placement OF content on your site, such as whether to put the main menu on the right or left side of the screen.
Using CSS within the template design, you can change the colors of your backgrounds, text, links or just about anything that you could within your ordinary (x)html code.
Images and effects
You can also affect the way images are displayed on the page in various ways, as well as more sophisticated tasks such as creating flash-like effects or creating Ajax applications such as drop-down menus.
The same applies to fonts. The designs for these are all set within the template's CSS file(s) to create a uniform look across the whole of your site, which makes it fantastically easy to change the whole look just by altering one or two files rather than every single page.
Browser specific solutions
A template can be designed to alter how it displays on different web browser to take full advantage of latest developments without making your site inaccessible to those who are not able to run "up-to-the-minute" system upgrades (such as certain companies who limit what software their employees can use).