The Sony Alpha 7R is a full-frame E-mount mirrorless camera. The A7R is very similar to its less-expensive sibling (the A7), except that it uses a 36 megapixel CMOS sensor with no optical low-pass filter. The A7 twins are the smallest and lightest full-frame cameras on the market.
While the cameras can use any E-mount lens, there will be a crop factor involved (unless you don't mind vignetting). Sony is producing a new line of FE-series lenses that take advantage of the larger sensor.
The A7's use the new Sony Bionz X processor, which produces images with better detail, less noise, and reduced diffraction. Other features include a 3-inch, tilting LCD, XGA OLED electronic viewfinder, Multi-interface (hot) shoe, Wi-Fi with NFC, and 1080/60p video recording.