Apple described Live Photos as 12-megapixel still images with data captured before and after the shutter button is pressed. Apple utilizes an "Efficient frame-to-frame technology" to store the additional movement.
Further information reveals that Live Photos combine 3 seconds of video (1.5 seconds before and after the still is captured) with the 12 megapixel image. There are 45 frames saved, equivalent to 15 frames per second.
The combined Live Photo file can take up almost twice the space as a single still image captured at the same resolution.