Zen Cart Templates
The Zen Cart Template is the presentation layer of your ecommerce software - it determines how your cart appears to browsers. Other systems call this component a theme or a skin. If you are not familiar with templates, then the Zen Cart documentation article What is a template?
is a good place to start.
I use a template called Bootstrap on my store
Zen Cart 1.5.5 and above have a built in mobile template called responsive_classic. You can see an example of it on Demo Site 5
Responsive Sheffield Blue
The author of the responsive classic template has another template that's very popular called Responsive Sheffield Blue. You can see an example of it on Demo Site 3
What if I like my desktop template?
Some customers love their desktop template and want to keep it, but want a responsive site too. My solution for this is to keep their desktop template, and use the responsive_classic mobile friendly template only when someone visits from a mobile device. I call this Responsive Fallback