Personal Ambient Displays are small, physical devices worn to display information to a person in a subtle, persistent, and private manner. They can be small enough to be carried in a pocket, worn as a watch, or even adorned like jewelry. In our implementations, information is displayed solely through tactile modalities such as heating and cooling, movement and vibration, and change of shape. The displays are designed to be ambient, transmitting information in the background of a person's perception through these simple, physical means. Studies are being done that examine how people perceive information through tactile displays, and application areas where ambient displays are appropriate are being identified.
(Source: http://tangible.media.mit.edu/projects/pad/)