The alternative minimum tax (or the AMT) -- which is hitting a growing number of Americans--is a separate tax computation that, in effect, reduces the benefit of certain deductions and credits, thus creating a tax liability for an individual who would otherwise pay little or no tax. The purpose of AMT is to ensure that wealthier taxpayers do pay some tax.
You may have to pay the alternative minimum tax if your taxable income for regular tax purposes, plus any of the adjustments and preference items that apply to you, is more than a specified exemption amount, for married and surviving spouses, the amount is $74,450 for 2011; lower amounts apply to other filing statuses). In other words, the AMT operates under many different rules than its counterpart.
AMT does not apply to every taxpayer who makes more than the exemption amount. It’s a combination of your itemized deductions, above the line deductions, and credits that can trigger AMT. A taxpayer whose deductions and credits fall within normal limits would be unlikely to trigger AMT. A taxpayer with extraordinarily high deductions and credits might trigger AMT.
To determine if you may be subject to the alternative minimum tax, see the Form 1040 instructions or refer to Form 6251, Alternative Minimum Tax - Individuals.