Starting with version 1.6, PrestaShop theme API heavily relies on the Bootstrap framework, together with Sass and Compass. Simply put, if you want to build a 1.6-compatible theme, you must use these technologies.
Bootstrap is "a sleek, intuitive, and powerful mobile first front-end framework for faster and easier web development." This tools makes it a lot simpler to build responsive web sites – meaning websites which adapt their design to the size of the screen, from mobile phone to giant TV.
A Bootstrap design is made of: