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

The main difference between a Roth 401k and a traditional before-tax 401k is how they are taxed. With a Roth 401k, contributions are made after taxes, so withdrawals in retirement are tax-free. In contrast, traditional before-tax 401k contributions are made pre-tax, so withdrawals in retirement are taxed as ordinary income.