This article explains the marginal rates:
So what's you're tax bracket?
Not every dollar of your income is taxed at the same rate.
That's because portions of your income fall into different brackets, which are assigned tax rates and increase on a graduated scale. Generally speaking, the first dollar you make will be taxed at a lower rate than the last dollar you make.
Remember, your taxable income is not the salary your boss told you you'd make when you got your job, but the amount of income left over after you've made your pre-tax contributions to your 401(k) and after you've subtracted the tax breaks to which you're entitled. (We'll talk about those breaks later, in "1040 Mysteries Revealed.")
The income ranges that define tax brackets are adjusted for inflation, change yearly and differ depending on your filing status (e.g., single, married filing jointly). Tax rates can change as well. In fact, under the Tax Relief Acts of 2001 and 2003, they have (see table). What's more, a new 10 percent tax bracket was added, reducing the amount of income that used to fall in the 15 percent bracket.
Here's an example of how income is taxed: Say you are single and report $80,000 in taxable income in 2003. In accordance with the income ranges defining federal tax brackets for single filers in 2003, the first $7,000 of your income is taxed at 10 percent; dollars $7,001 through $28,400 are taxed at 15 percent; dollars $28,401 through $68,800 are taxed at 25 percent; and dollars $68,801 through $143,500 are taxed at 28 percent.
When people ask you what your tax bracket is, they're really asking for your marginal tax rate. That is, the percent at which the highest portion of your income is taxed. In the example above, if you report $80,000 of taxable income in 2003, your marginal tax rate is 28 percent -- the rate at which the last dollar of that $80,000 is taxed. But your effective rate is the overall percentage of your taxable income that was actually paid in income taxes at the end of the day. And that rate may be lower than your marginal tax rate.
You should also be aware of what's known as your combined tax bracket. That's the sum of your federal tax bracket and your state tax bracket, minus the amount of state taxes you can deduct from your federal return. (Example: If your top federal rate is 28 percent and your state tax rate is 5 percent, your "effective combined rate" is 33 percent.)
That number will determine how much tax you'll owe on income from your investments. (If your combined bracket is 33 percent, then 33 percent of your investment income will go to the government. Put another way, you'll be able to keep 67 percent of your investment income.)
http://money.cnn.com/pf/101/lessons/18/page2.html