This was one of the first responsive sites I built (after the one you’re on and a personal blog), in 2013. It was build with WordPress, which keeps getting better and better, and was designed by Daryl McCool at d.a.m. Cool graphics.
“Responsive” websites change according to the capabilities of the device being used to view them — a desktopp computer, say, or a phone or a iPad. Usually, responding to the screen size of the device is all that is necessary, and the same content is delivered to all devices, but presented differently. The development strategy I favor is Mobile First, which begins with a site suitable for the least-capable device then progressively enhances the user experience as more capable devices are introduced. This adds complexity, and so cost, to a web project, but as web-capable devices proliferate, device-responsiveness is becoming a necessary feature to include.