What is the difference between pretax and Roth 401(k) contributions?

The main difference between pretax and Roth 401(k) contributions is how they are taxed. Pretax contributions are taken from your paycheck before taxes are deducted, reducing your taxable income now but you will pay taxes on the withdrawals in retirement. Roth contributions are made after taxes are deducted, so you won't pay taxes on the withdrawals in retirement.