What are the differences between contributing to a 401k pre-tax versus contributing to a Roth 401k?

Contributing to a traditional 401(k) is done with pre-tax money, meaning the amount you contribute is deducted from your taxable income for the year. This can lower your current tax bill. On the other hand, contributing to a Roth 401(k) is done with after-tax money, so you don't get a tax break now, but your withdrawals in retirement are tax-free.