What is a Roth 401(k)? 
 Roth 401(k) is a type of 401(k) where accounts, contributions, and withdrawals have no impact on income tax. The benefit of the Roth account is that the capital gains are tax-free.

In the United States, a 401(k) plan is an employer-sponsored, defined-contribution, personal pension (savings) account, as defined in subsection 401(k) of the U.S. Internal Revenue Code. Periodical employee contributions come directly out of their paychecks and may be matched by the employer.