If you are in the 25% tax bracket, that means your last dollar of income is taxed at 25%.
Taxable income is equal to gross income minus deductions and exemptions. So, if your taxable income is zero or less, you don't pay any tax. If you have gross income in excess of your deductions and personal exemptions, then the first dollars above that amount are taxed at 10% - in the 10% bracket.
For 2005, the 25% bracket (for single taxpayers) consists of taxable income between $29,700 and $71,950. The amount of your taxable income that is more than $29,700 but not more than $71,950 is taxed at 25%. As long as your taxable income stays under $71,950, the next dollar of income will also be taxed at 25%. Income over $71,950 will be taxed at a higher rate -- either 28%, 33%, or 35%. The 35% bracket applies to taxable income in excess of $326,450! May we all be so fortunate as to pay tax at the highest rate!