I was asked this very question this week, so I thought if one person still needed to know, there were more. If you don’t feel like cruising the infographic, I can answer the question for you this way.
In the past you had a website that looked fine on laptops and desktop computers, but looked terrible on a tablet or phone. To keep readers happy on phones and tablets, you had what’s called a mobile site. This is as separate, stand-alone, very friggin’ expensive website that was automatically to served readers when browsing on their phones.
A responsive site is a site that recognizes what type of device the reader is viewing your site on, and then adjusts the text, images and other elements of the site to fit their screen. It’s important to make sure your site is responsive since almost 60% percent of website traffic is via mobile these days.