A Roth 401(k) is available to anyone whose company offers it; for 2013, 401(k) plan participants can contribute up to $17,500 ($23,000 if you are at least 50 years old). Qualified Roth 401(k) distributions are tax free while distributions from a 401(k) are fully taxable (since their contributions were tax deductible).
Unlike a Roth IRA, RMDs from a Roth 401(k) must be taken if you are at least age 70 ½; however, a Roth 401(k) can be directly rolled over (or use a trustee-to-trustee transfer) into a Roth IRA. This type of direct rollover (or transfer) is not a taxable event. Thus, someone with a Roth 401(k) account can avoid any kind of lifetime distribution by moving the money to a Roth IRA.