What are the differences between a traditional and Roth 401k?

The main difference between a traditional and Roth 401(k) is how they are taxed. With a traditional 401(k), contributions are made with pre-tax dollars, reducing your taxable income now but you pay taxes on withdrawals in retirement. With a Roth 401(k), contributions are made with after-tax dollars, so withdrawals in retirement are tax-free.