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.
Bootstrap
I use a template called Bootstrap on my store.
Responsive Classic
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.