What, exactly is "responsive design," and why does it matter in marketing? As Google describes it, it is a website design technique that allows you to create a single site that adapts to the device on which it’s being viewed, whether it’s a laptop, smartphone or tablet.

So, rather than seeing a tiny, little and unreadable version of a website over a smartphone, the user sees a smartphone-optimized website. The idea is that the webmasters need not build several different versions, each with its own URL.

So a site built with responsive design automatically resizes for different devices, but "it is up to you to prioritize the content that matters most to the mobile user," advises Google. A smartphone user may need phone numbers or directions quickly, but tablet users might want a purchasing interface front and center. versions. A site built using responsive design could prioritize click-to-call and click-to-map buttons, while the tablet site would focus on simplifying the shopping cart.

If responsive design is all that easy, why would anyone do otherwise? As Tripwire magazine described in an article about WordPress themes (in an article comparing WordPress mobile versus responsive themes), a responsive theme may be simpler for maintenance; but a mobile-only theme offers complete control of the mobile experience, without any impact upon the main site design. And a well suited WordPress mobile plugin can enable the masters to ad mobile-site only special features and user experiences.

Google offers a side-by-side comparison (see graphic), and some practicalities:

• A separate mobile optimized site typically has a separate mobile URL, but users will be taken there automatically as long as the desktop site is enabled with the auto redirect code.
• Prices vary across developers and agencies for responsive design.
• A separate mobile site is more do-it-yourself, faster to create and probably a richer, more elegant interface. But it requires separate updating from the desktop site—a two-step process versus one-step.
• Google did not offer figures, but estimates that a separate mobile site is less costly to develop.