The small dongle features a USB port for power and an HDMI port for output. The device acts as a WiFi hotspot that your HTC One X/S connects to (the WiFi radio on the phone can also be connected to a WiFi AP while working with the Media Link HD). Once paired, you can use a three finger swipe gesture on the phone to either mirror or use any HDMI display attached to the Media Link HD as a secondary screen. HTC has an API that it will make public at some point in the future, allowing 3rd party devs to take advantage of the dual-screen feature of the Media Link HD.
Similar to other wireless display technologies, you're actually sending an encoded stream of the phone's framebuffer to the receiver where it's decoded and sent over HDMI. No word on pricing or availability of the Media Link HD.
Although this sounds a lot like WiFi Direct/WiFi Display, the link is similar but proprietary to HTC. Eventually HTC will support WiFi Display but we're still several months away from that being the case (at the earliest), and it's unclear as to whether or not the Media Link HD will be updated to support it or if you'll need a new receiver.