No - only part of your income that is above certain level will be taxed at higher rate - not entire income.
Tax brackets are depend on the filing status.
You may see applicable tax brackets and examples here - http://www.moneychimp.com/features/tax_brackets.htm
Let me know if you need help with extimating your tax liability or withholding.
Your tax liability is calculated based on the taxable income.
(Taxable income) = (adjusted gross income) - (deductions)
See for instance tax schedule for 2008 in the instructions for the form 1040 - http://www.irs.gov/pub/irs-pdf/i1040.pdf - page 92.
Assuming - you are single with taxable income $50,000 - your tax liability will be $4,481.25 + 25% on the amount above $32,550
because your 25% tax bracket is started from $32,550.
on the amount above $78,850
because your 28% tax bracket is started from $78,850.