Oh, that's a good question, because it is a source of much confusion. The U.S. income tax code is called progressive because tax rates increase as income rises. For example, in 2013, income up to $8,925 was taxed at a rate of 10%. Income from $8,926 to $36,250 was taxed at 15%. Income from $36,251 to $87,850 was taxed at 25%. It is said that someone earning $50,000 falls into the "25% tax bracket." Seems pretty simple, right?
Wrong.
When we say someone is in the "25% tax bracket," it sounds like all $50,000 of their income is taxed at 25%, which would result in $12,500 in taxes. But that’s not true. Only the money exceeding $36,250 is taxed at 25%. The rest is taxed at 15% and 10% according to the amounts we just listed [source: Fidelity]. The actual tax owed by someone filing for $50,000 is $8,428.75. The tax rate is 16.8%. (The real rate would be significantly lower after deductions and exemptions.)
The term "tax bracket" is meaningless. The correct term is "marginal tax rate." This is the tax rate applied to the last dollar you earned during the tax year — 25% in our example above. The real rate you pay to the IRS — 16.8% in our example — is called the "average" or "effective" tax rate. The average rate is always lower than the marginal rate unless you earn $8,925 or less, in which case they are equal.