Lots of people will argue the fine points of this (there are whole forums dedicated to it, really) but here are the basics. Both accounts have tax benefits - just different kinds.
In a Traditional IRA, you fund the account with pre-tax money and then pay taxes when you withdraw. So if you made $55,000/year and put $5000 into a Traditional IRA, you'd claim it on your taxes and only pay tax on $50,000 of income. Then when you withdraw money from the Traditional IRA, it counts as taxable income and you pay taxes on it like usual. This is good if you are in a high tax bracket now, and you think you will be in a lower tax bracket later. It's usually difficult to take money out of a Traditional IRA without paying a penalty.
In a Roth IRA, you fund the account with post-tax money and then pay no taxes when you withdraw. So if you made $55,000/year and put $5000 in a Roth IRA, you'd still pay taxes on all $55,000 of your income, but then when you go to withdraw the money, you don't need to pay any tax. This is good if you're currently in a low tax bracket and think you might be in a higher one later; it's also good because there are fewer restrictions on when you can take the money out (you can take your contributions out at any time - earnings have some different rules about them).
Clear as mud? :)