Human: What is the difference between a Roth IRA and a 401k? Assistant: 
The main difference between a Roth IRA and a 401k is that contributions to a Roth IRA are made with after-tax dollars, while contributions to a 401k are typically made with pre-tax money. With a Roth IRA, you pay taxes when you withdraw your money in retirement, but not when you contribute. With a 401k, you pay taxes when you contribute, but not when you withdraw.