A tax bracket is a range of incomes taxed at a particular rate. The U.S. uses a progressive system with 7 brackets (10%, 12%, 22%, 24%, 32%, 35%, 37%).
Common Misconception
Moving into a higher bracket does NOT mean all your income is taxed at the higher rate. Only the income within that bracket is taxed at that rate. Your overall effective tax rate is always lower than your marginal (bracket) rate.
Example
A single filer earning $50,000 pays 10% on the first $11,925, 12% on $11,926–$48,475, and 22% on the remaining $1,525. Effective rate: ~13%.