Smart phones are ubiquitous now. A large percentage of web browsing is viewed from a mobile device such as an iPhone or Android device. It is important that your business web site is developed for these devices first, then the other web devices will follow.
Responsive web design is critical on the internet now. Your web site is designed to look great on any web enabled device, regardless of screen size and resolution.
Tablets, such as iPads, are gaining in popularity as a web browsing device. Rest assured, your web site looks great on this as well.
Of course, your web site looks great on a desktop PC/Mac.