Responsive design allows you modify the layout, navigation, content, and appearance of a website or application based on the width of the user’s browser.
Example: This website uses responsive design — if you’re viewing it on a resizable browser (laptop/desktop), see what happens when you narrow the width of your browser. If you’re using a tablet browser or smartphone, try changing its orientation, e.g., from landscape to portrait. At the smallest screen sizes, the top navigation turns into a dropdown and multicolumn layouts become one column, all done through the magic of media queries and flexible layouts.
Some people think responsive design is the one-and-only answer to sites and applications for mobile users. Others believe it’s best to have separate websites or apps for different devices, e.g., iPad, smartphones, and laptops/desktops.
I’ll go with the classic usability answer: it depends.
When not to go with responsive design
Think about adding a mobile solution if your users’ needs change when they change devices or locations. For instance, a mobile user may be more interested in getting a fast answer (where’s the baggage claim for this flight) then on doing comprehensive research (I want to plan all facets of my vacation).
You may need a mobile app to accommodate users who aren’t connected. I prefer reading The New York Times online in my browser. But I rely on The Times’ iPad application when I’m waiting in my doctor’s office, because I can’t connect to the Internet there and the app automagically downloads each day’s paper to my iPad. Nice for me on occasion; perfect on a daily basis for commuters.
Small screens are better at consuming information than creating it. An expense system for a smartphone might focus on letting a user work on one item at a time because that’s all there’s space to display. A larger screen provides the luxuries of seeing that item in context with your other expenses and working with more than one expense at a time.
Responsive design can be a great answer
Often, especially for content-oriented sites, the mobile experience can and should be very similar to the laptop/desktop experience. This is where responsive design can truly shine. Most content is created once without much consideration about how it will appear on different devices — the underlying architecture of the website takes care of that. The result is a great experience on whatever the device the user has without the extra work of maintaining multiple sites and applications. UX nirvana.