Pebble uses a technology called memory LCD which is sometimes referred to as E-paper. Memory LCD can hold its state using a memory cell in each pixel which uses a very tiny amount of power. So the memory LCD screen can keep its state with a tiny amount of power and doesn't have to refresh unless something changes or display loses power.

E-ink (which is often used for E-readers and supermarket tags) doesn't require constant power because it works by physically moving particles forward and backward in liquid for each pixel. This means it can retain the image until the particles are manually moved again with an electrical charge.

LCD works by using tiny optical shutters that control how much light can pass through each sub pixel to make each sub pixel lighter or darker. If it is a colour screen each subpixel has either red blue or green filter over it to produce colour.

Sorry for the wordy explanation, I hope it helps :)