The Alternative Minimum Tax is the tax that is calculated for individuals and not corporations. It is what you pay to the IRS when you file your federal income tax return.
If you are under this tax, the IRS has the right to demand your return, and you will not be able to file another return or claim deductions until the IRS is satisfied that you have paid all of your taxes on time.
It is very important to understand that the Alternative Minimum Tax is not a "big" tax, but rather it is the small business owner's best friend.