In short, it's the same Switch, but with unnecessary features removed. Lite model doesn't have dock or removable joycons. There will be 3 colors: yellow, cyan and grey. The most important thing of lite is that it will cost sensible 200 dollars. That's 100 dollars less, while not really losing anything truly important.
You will be able to buy it in 2019.09.20 (at least in Japan).
Here's a more complete change list: