With Roth contributions, you make contributions with after-tax dollars. Earnings are free from federal taxes if you take a qualified distribution. A "qualified distribution" is one that is taken at least five years after the first day of the year of your initial Roth contribution, or Roth conversion, if earlier, and after you have reached age 59½ or become disabled or deceased. If you take a nonqualified withdrawal, any investment earnings on the Roth contributions are subject to regular income taxes, and you may be subject to a 10 percent additional federal tax if you withdraw such earnings before age 59½ unless an exception applies. State income tax laws vary; consult a tax professional to determine how your state treats Roth distributions.
With pre-tax contributions, you get a tax break now, reducing your current taxable income. Taxes on contribution and earnings are due upon withdrawal.