Responsive web design (RWD), the ability for your web application to “respond” to the size of the viewport (traditional screen size vs mobile screen size) is a commonplace practice today. Responsive design leverages client side, front-end UI frameworks to hide, show, stack, shrink and grow elements on the screen in a way that is appropriate for the size of the device’s screen. In essence, it is many user interfaces in a single code base.
Adaptive web design (AWD), a (seemingly) alternative approach to RWD is a server-side capability to respond to the client with completely unique templates/markup for a given device type (amongst other facets). Adaptive design, while extremely powerful, is somewhat less common in part because it relies on a server and is thus a less general technology. It’s also less prevalent in part because, for a large class of problems, RWD is “enough” to get the job done. However, as complexity and sophistication of requirements mount, adaptive design starts to show clear advantages over a purely responsive approach.