The HD Texture Pack is an in-app-purchase introduced in update 1.4. It changes the appearance of all blocks, crafting surfaces, Blockheads, and items to hand-painted, high-quality, versions. Like Double-Time, its effects apply in all worlds, including online servers, accessed by that device.
The feature can be purchased and restored in the Options menu of the Pause menu. It can be restored onto multiple devices if they use the same iTunes, Google Play, or Amazon account. Once purchased or restored, they can be turned on and off for that device freely.