Although people are sometimes confuse designated Roth accounts in qualified retirement plans with Roth IRAs, both are referred to in the tax code as Roth accounts, just different types of Roth accounts. So it's accurate to call this type of account in a qualified retirement account like a 401(k) a Roth account ("designated Roth account" is the proper term). Using the term "Roth 401(k)" is a bit of a misnomer because the Roth account in the 401(k) is not in a separate plan from the plan which has the traditional account in the 401(k); "Roth 401(k)" simply refers the designated Roth account in the 401(k).
The rules for making contributions to and distributions from Roth IRAs and designated Roth accounts in qualified retirement plans are substantially different. The things that they have in common and make them both types of Roth accounts are that contributions are made after-tax and earnings are tax-free once the requirements for qualified distributions are met.
