You've probably heard about responsive websites or responsive design, but if you don't live in the world of web design, you may not know what the term "responsive" means in this context. It simply means that the a website "responds to" the physical size of the device it's being displayed on by adapting its layout and other aspects of its design.
When smartphones first became popular, the concept of responsive website design hadn't been invented yet, and so the common solution was to build and maintain two websites — one for computers and one for smartphones. Responsive design relieved us of this requirement and allows us to build a single website that works everywhere.
All of the websites we build are responsive, as is this website you're reading right now. To see how it adapts to different screen sizes, try resizing the browser window to make it narrower or wider, and watch how things change. (If you're on a smartphone, try rotating the device.)
Our websites are designed to work on all device sizes and are specifically optimized for the most common sizes: smartphones, tablets, laptops, and desktops.