What are the differences between contributing to a pretax 401k and an after tax 401k, and how do these choices impact my retirement savings?

Contributing to a pretax 401k means you don't pay taxes on the money you put in now, but you will pay taxes on it when you withdraw it in retirement. Contributing to an after-tax 401k means you pay taxes on the money now, but won't pay taxes on it when you withdraw it in retirement. The choice impacts your retirement savings by affecting when you pay taxes on the money and how much you ultimately have available for retirement.