To do a car analogy (this is Slashdot, right?):

With Ubuntu, every time you want to fix something with your car, you roll it into the garage, pop open the hood and get to work. It's intensive labour, results will vary, and undoing a change can be really difficult.

With NixOS, it's like 3D printing a new car every time. You'll design a model, press a button, and the car gets built from scratch. If you don't like it, tweak the design a bit, and print a new car. If the new car breaks, just go back to the previous known-good one, which is already in your garage. You can even take the design documents to your friend and generate an exactly identical model.