A mobile theme will morph the site based on the screen size. CSS3 media queries are used to detect the device size and breakpoint will adjust the design based on CSS rules. Twitter Bootstrap and Zurb Foundation are the two largest responsive frameworks. I beta tested the PageLines framework yesterday. This group is on the right track as far as how they envision the web building paradigm shift. RWD will server both the mobile user and desktop user from a single code-base. I love it. Some hate it. It is the "new thing" in webdev.