What is the difference between a Roth 401k and after-tax contributions?

The main difference between a Roth 401(k) and after-tax contributions is how they are taxed. Roth 401(k) contributions are made with after-tax money, meaning you pay taxes on the money before you contribute it. After-tax contributions are made with money that has already been taxed, so you won't pay taxes on that money again when you withdraw it.