An individual retirement account funded with after-tax money that grows and is withdrawn tax-free.
A Roth IRA is funded with money you have already paid tax on, so qualified withdrawals in retirement are entirely tax-free, and contributions (not earnings) can be withdrawn any time without penalty. Eligibility to contribute directly phases out above certain income levels. It has no required minimum distributions during the owner's lifetime.
Decades of tax-free growth make a Roth IRA especially valuable for younger savers.