A Traditional IRA is tax deductable. You are allowed to deduct the amount you put into a Traditional IRA the year you put the money in (well, actually up to the tax deadline of the following year, so for 2011 you could deduct it on your 2011 taxes as long as you put your contribution in by April 17, 2012). When you take the money out during retirement you will be taxed at your current level.
A Roth IRA is not tax deductable. You include the income (that you then put into the Roth IRA) in your taxable income for the year you made it. When you take out the money during retirement, though, you do not pay taxes on that income.
Both ways you are taxed once on the income. With a Traditional IRA you are betting that you will be in a lower tax bracket during retirement. With a Roth you are betting that tax rates will have increased by the time you are in retirement.
Is that enough information for your needs?
Here is a quick overview: