Electric paper was first developed at the Xerox Palo Alto Research Centre in the 1970's, the same place ( and time ) that gave us windowed PC's screens with mouse pointers.

As with PC technology, others have taken the raw technology and run with it.

Today we see Kindles, Nooks, and now Kobo in New Zealand. Whitcoulls recently ran out of their initial supply, and have had to hurriedly import more of the devices to meet the demand.

There are various technologies used and here is a short description of each.

Gyricon

Uses charged polyethylene spheres black on one side, and white on the other, forming a dipole. The spheres are suspended in oil allowing them to move freely, and are flipped between the black and white state by applying a positive or negative voltage.

Electrophoretic

In an electrophoretic display, charged particles of a product such as titanium dioxide as held between two plates. When voltage is applied to different parts of the plate, the particles move to teh front or the back. When at the front the screen appears white, and when at the back, the screen takes on the color of the suspending fluid, usually dark.

Electrowetting

Electrowetting displays utlise an interaction between water and oil and the effects of applying a voltage to an insulated electrode. This technology is especially exciting as it allows a colour display that can change quickly enought to support video content.

Electrofluidic

Electrofluidic displays are a variation on Electrowetting.

There are other display technologies under development. For more details check out the Wikipedia Article here.