Contributions to a Traditional 401(k) plan are made on a pre-tax basis, resulting in a lower tax bill and higher take home pay. Contributions made to a Roth 401(k) are made on an after-tax basis, which means that taxes are paid on the amount contributed in the current year. The reverse is true once you are eligible to make 401(k) withdrawals. Withdrawals from Traditional 401(k) plans are taxable, while those made from a Roth 401(k) are not.
Traditional 401(k) vs Roth 401(k)