You don't get a tax deduction for money you put into a Roth IRA, like you do with a traditional deductible IRA. However, your money grows tax free and generally remains tax free when you take it out, assuming you meet certain restrictions.

You can withdraw an amount equal to your contributions tax-free at any time, because you have already paid tax on this money. You can withdraw your earnings tax free in the year you reach 59.5, as long as the account has been open for at least five years. For other withdrawals, complicated rules apply. (See http://www.rothira.com/disttax.htm.)

With a traditional IRA, every dollar you take out is taxed as ordinary income.

One advantage of a Roth IRA is that you do not have to start taking money out after you turn 70.5, like you do with a traditional IRA.

You cannot contribute to a Roth IRA if your income exceeds certain limits. For 2007, the phaseout range is $156,000 to $166,000 for married couples filing jointly and $99,000 to $114,000 for singles.

For more on Roth IRAs see http://www.irs.gov/publications/p590/ch02.html