A website design can be mobile-friendly without being responsive.
I’ve always taken the distinction for granted until I was recently doing a website review for a new client. I noticed they were using WP Touch, a WordPress plugin that creates a mobile-friendly version of a website. This puzzled me because the theme they were using was responsive – that is, it displays well on any type of device.
It turned out the client didn’t realize that a responsive design means a site is mobile-friendly, so they thought they needed a plugin to make the site work on smartphones and tablets.
The most important point, though, is whether or not you use a responsive design, make sure your site is mobile-friendly.