Twenty Twelve is a theme bundled with WordPress and part of project to have new default theme every year. It is fully responsive to look great on every device. It was first bundled with WordPress 3.5.
It also supports some of the latest (at the time) native WordPress features such as a front-page template with custom widget areas, an option to use the Open Sans webfont, styling and support for post formats, and custom page templates, as well as classic features such as custom background, menu and header.
Twenty Twelve also uses many of the theme development standards such as escaping all output, custom page templates in their own directory, flexible title tag, responsive layout and navigation, support for the theme customizer that was introduced in WordPress 3.4, and works right out of the box, with no Theme Options page. Twenty Twelve can easily be extended and customized with the help of child themes.