Electonic ink, or e-ink, turns a piece of paper into a computer monitor. Imagine you've just finished reading Moby Dick. Press a few buttons on the spine of the book in your hand, and the words on its pages change to The Count of Monte Cristo.
Electonic ink is a vague term, just as electronic book can mean anything from a PDF file the hardware that displays such files. Most e-ink designs use electronic "paper"- a plastic film filled with tiny magnetic pigment capsules. Electrical charges are used to arrange the capsules and show content or reformat the display (think of a Boggle cube if that helps).
Companies working on e-ink include E Ink, a pre-IPO Bell Labs spinoff, and Gyricon Media, a division of Xerox PARC. Some early prototypes were dynamic signs in stores - not much more than large flat panel displays. Recent efforts have produced some thin, flexible models, some even in color. While you're waiting for e-ink and the flying car, it's probably best to check out A Tale of Two Cities from your local library for now.