From Matthew Panzarino's review of the iPhone 6s over at TechCrunch:
Live Photos are not really a new format. The images, which are accompanied by 3 seconds of video (split before and after your shot) are stored as a .jpg file on your iPhone. The video is a .mov file containing 45 frames that play back at around 15fps when you press and hold on an image. The whole package takes up roughly the space of two regular 12 megapixel images. The appearance to the user is seamless, as iOS sees that they are connected, and presents them as one ‘Live Photo.’
In my experience, Live Photos work best when capturing ambience, not action. Because the frame rate is relatively low, moving the camera a ton while you shoot them or having a subject move will display a bit of jitter. If, however, you’re shooting a still image with some moving elements, the effect is extraordinary.
Review: Apple’s iPhone 6s And 6s Plus Go ‘Tick’ | TechCrunch
I ran into this at MacRumors, where they point out that Live Photos imported into Photos on El Capitan work like you'd expect, a single JPEG in your library that plays as a movie when double clicked. Imported into Photos on Yosemite however splits the file into its two components, 1 JPEG, and 1 MOV file, which just seems weird (limited by OS instead of application), but also interesting (the gentle nudging into new hardware and new software).