In more detail
You pay tax on the contribution now and none on qualified withdrawals, which generally means after age 59½ and at least five years after your first Roth contribution. Your own contributions (not the earnings) can be taken out at any time without tax or penalty. There are no required minimum distributions during the owner's lifetime. Income limits may restrict who can contribute directly, and the yearly limit is shared with any traditional IRA. Money can also be moved in by converting a traditional IRA, paying tax on the converted amount.
Why it matters
If your tax rate is the same later, a Roth and a traditional account end up identical; a Roth wins if your rate rises and loses if it falls. Tax-free income in retirement also does not push up the tax on other income.
Example
A $6,000 yearly contribution at 7% for 30 years grows to about $566,765, of which $180,000 is contributions and $386,765 growth, all of it withdrawable tax-free once the qualified-withdrawal rules are met.
Common confusion
A traditional IRA gives the tax break now and taxes withdrawals; a Roth IRA taxes contributions and frees withdrawals.