From the article:

Many website owners say to themselves, “I want my site to look great on mobile, but I don’t know where to start.”

If you are in the business of building and designing websites, you cannot ignore the fact that many people are going to be visiting your sites on their smartphones and tablets. The Web and the mobile browsers remain one of the top ways that users interact with websites and if they have trouble on their smartphone, there is a good chance they are not coming back.

That’s where responsive design can help.

Responsive design is a concept where you build your website once and then format it so it can adapt to any screen size that accesses it. Designers use HTML5 and CSS to build the sites and set parameters so the content will resize itself whether the user is in vertical or horizontal viewing mode, on a tablet, desktop or smartphone or even a screen as large as a television.

via 10 Developer Tips To Build A Responsive Website [Infographic] – ReadWrite.