An reader, such as a Kindle, a Kobo, or a Sony Ereader, uses a technology to display the text on the screen without a light. Other devices, such as an iPad, a cell phone, your computer, etc. use a lighted display.
The difference is this - ereaders need light shone on them (like a real book) whereas the other devices have an internal light. I refuse to use the technical terms so I am going to refer to one as "etch-a-sketch" and the other as "Christmas lights".
With a "Christmas Light" device, you can read in the dark. The computer you are reading this on right now is a "Christmas Light" device. So is your iPhone, your iPad, your Android, well, everything except ereaders.
The display on an ereader, like my Kobo, does not light up. To read it, you have to hold it under a light the same way you do with a real paper book. The screen operates kind of like an etch-a-sketch, but in a much more sophisticated way. The image you see on the screen isn't made up of pixels of light, it's made of tiny pieces of metal that are electrically charged. So, the only electricity that an ereader needs is to turn pages.
The "etch-a-sketch" screen (which, to be fair, is called "e-ink") is much easier for human eyes to look at for long periods of time than the "Christmas Light" screen (which is LED, or LCD or lots of other things). Right now it only works in black-and-white, but color is on it's way. This is an exciting time for this new technology. It looks pretty cool now, but in the future it will be like black-and-white TVs, pretty primitive.
As you can tell, I like cutting-edge technology, and especially if it works for me. If you would like to learn Adobe Photoshop, Illustrator, or Dreamweaver, please contact me. I do personal on-site training sessions in the greater Phoenix, Arizona area.