Magento template system allows you to easily create custom html/css code according to design requirements. However, in some cases it would be a good idea to use some of the default magento templates. It may save some time spent on integrating new html structure with magento functionality.
For example, let's take a look at this location in standard magento structure: app\design\frontend\default\default\template\page. Here we have templates describing a basic structure of a page: 1column.phtml, 2colums-left.phtml, 2colums-right.phtml or 3colums.phtml. They all contain the same structure of the page (let's focus on the inside of 'body' section):
<!-- start header -->
<!-- end header -->
<!-- start middle -->
<!-- end middle -->
<!-- start footer -->
<!-- end footer -->
They differ only in part inside
<div class=”middle-container”>, where a html code for specified layout is defined. As you can see, they present the most common way of building the pages – header, middle part, footer. So in most cases you can use those templates without any change in html and create only your own skin css, which will adapt the look of your page to your needs.
Of course, the more detailed a template is, the less chance that it can be used without change, but you can still start working on a default magento template and just modify the parts you need.