With more and more mobile phones, tablets & smart devices, there is a sudden shift in the way people use & view the web pages. These days most people prefer to view the websites & web pages on their smart phones & tablets. Creating separate websites for mobiles/tablets is an out-dated idea these days since the mobile browsers are getting smart and are capable of displaying even the complex content from the webpage.
Responsive Web Design is the way to go to bridge the gap between the PC/mobile based web designs. A response website design or theme is an approach through which a website breaks itself down smoothly across different screen resolutions, different monitor sizes, on resizing of the browser window. The responsive website design/theme also caters to different platforms, computers, tablets or mobile phones. Using Response Website Design, a developer can create websites which are optimized for multiple platforms & resolutions in terms of navigation, readability, usability & load time.
There is multiple numbers of times when you have faced that resizing a window breaks the website layout and hence leads to a bad user experience. Using responsive website design, this problem can also be solved.