They're a little different. Kettles are small (1-2 liters) will heat water until it's boiling and then shut off(or have the user disconnect the heat source)

Water boilers hold a larger amount of water (3-5 liters) at a consistent temperature with a button to dispense it.

I upgraded from a kettle to a zojirushi water boiler and I've never looked back. The thing is incredible. Absolutely worth the price.