What is the difference between a pre-tax and Roth 401(k) plan?

The main difference between a pre-tax and Roth 401(k) plan is how they are taxed. In a pre-tax 401(k) plan, contributions are made before taxes are taken out, reducing your taxable income in the present. In a Roth 401(k) plan, contributions are made after taxes are taken out, but withdrawals in retirement are tax-free.