E ink, or electrophoretic ink, is a technology used with electronic paper displays (usually ebooks) that displays text in a fashion that mimics physical paper and ink. It's a bistable process, meaning when the source of power is taken away, the display will remain unchanged. Instead of an LCD display, which requires backlighting, e ink is used with ambient light. E readers have a much longer lifespan on a charge than LCD screens due to the lack of backlighting.
E ink particles are microcapsules of black and white ink. They are electronically charged and respond to the correct configuration with the load of each page.
It was invented by Joseph M. Jacobson, an MIT media lab professor, and Barrett Comiskey, an MIT undergrad, between 1995-1997. The technology was well funded, extremely successful, and is widely used with e reading devices. Color e ink has been explored, but with less vibrant and therefore less popular results than LCD display e reader technology (like iBooks on the iPad) due to the nature of e ink.