What is Responsive Web Design?
Responsive web design is a way of making your website's design layout respond to the window size of the user. As the window gets smaller the content shifts around. This makes the website friendly for smartphone users and tablets.
The only other option is to have two websites or three websites. One for mobile phones, one for tablets, and one for desktops. If you have the money and the time, this second option is still the best for some companies. Feature phones are not smartphones. In Europe and some other countries feature phones are still heavily used, although in the last year it's dropped 20%. But if your company is primarily based in North America there is no reason to spend more money and need to care for multiple websites.
For WordPress users, there are many templates that are ready made to be responsive. Just be sure it says the entire template is responsive and not just one of it's features, such as a slideshow. My website design partner doesn't do non-responsive! It's just not how new websites are built. Even if you have deep pockets and staff, your main website should still be responsive! Not everyone browses the web with a full window. Use a responsive web design today and when features phones finally bite the dust then you can cut your overhead and make more money by deleting the obsolete sites.
While this isn't new news, it's now very hot news. The debate of whether to ignore mobile, have two sites or go responsive seems to be over. Feature phones are dying. Smartphones and tablets combined are over 20% of the internet users. Google says have a FAST responsive web design.