Responsive Web Design is the approach that allows your website or web app to respond to the user’s behavior and environment based on screen size, platform and orientation.
The implementation of Responsive Web Design is based on number of factors including coding flexible HTML grid layouts, using auto-resizable images, CSS media queries, dynamic measurement units for font sizes, such as “rem”, “%”, and of course thoughtful design planning.
You can implement everything by yourself from scratch. That would be, of course, a more involving task. Or you can use freely available resources on the web, such as Bootsrap framework. By loading a few extra files into your website and following their well documented rules, such as using special CSS classes for grid layout, and dynamic image resizing property in CSS, adding and customizing media queries, your site will transition into a magically fluid tapestry that will automatically adjust based on screen size. Bootstrap offers even more. I choose to use Bootstrap for rapid prototyping, as it provides basic, consistent and good looking modules that saves me time otherwise spent on trivial decorative tasks that would be later scrapped, anyway.
Implementing Responsive Web Design is not a simple task that can be completed overnight by adopting some magical plug-in. The process requires to revamp your existing code and it takes time. However, the payoff of keeping and gaining more customers that can access your site from any device, by far, worth the effort.