IRS Enforced Penalties :

The Internal Revenue Code imposes many different kinds of penalties, ranging from civil fines to imprisonment for criminal tax evasion.
If you do not file your return and pay your tax by the due date, you may have to pay a penalty. You may also have to pay a penalty if you substantially understate your tax, understate a reportable transaction, file an erroneous claim for refund or credit, or file a frivolous tax submission. If you provide fraudulent information on your return, you may have to pay a civil fraud penalty.
Penalties are generally payable upon notice and demand. Penalties are generally assessed, collected and paid in the same manner as taxes. The notice will contain the name of the penalty, the applicable code section, and how the penalty was computed (or information on how to obtain the computation if not included).
This fact sheet is the 22nd in the Tax Gap series. It provides additional guidance to taxpayers regarding civil penalties and the consequences for understating income and overstating expenses.
Estimated Tax-Related Penalties
Employees have taxes withheld from their paychecks by their employer. When you have income that is not subject to withholding you may have to make estimated tax payments during the year.
This includes income from self-employment, interest, dividends, alimony, rent, gains from the sale of assets, prizes, and awards. You also may have to pay estimated tax if the amount being withheld from your salary, pension, or other income is not enough to pay your tax liability.
Estimated tax payments are used to pay income tax and self-employment tax, as well as other taxes and amounts reported on your tax return. If you do not pay enough through withholding or estimated tax payments, you may have to pay a penalty. If you do not pay enough by the due date of each payment period you may be charged a penalty even if you are due a refund when you file your tax return.
Penalties for filing or paying taxes late
The most common penalties are for filing late or paying taxes late.
Filing late: If you do not file your return by the due date (including extensions), you may have to pay a failure-to-file penalty. The penalty is usually 5 percent for each month or part of a month that a return is late, but not more than 25 percent. The penalty is based on the tax not paid by the due date (without regard to extensions).
If you file your return more than 60 days after the due date, the minimum penalty is $100 or, if less, 100 percent of the tax on your return.
Paying tax late: You will have to pay a failure-to-pay penalty of ½ of 1 percent (0.5 percent) of your unpaid taxes for each month, or part of a month, after the due date that the tax is not paid. This penalty does not apply during the automatic six-month extension of time to file period if you paid at least 90 percent of your actual tax liability on or before the original due date of your return and pay the balance when you file the return.
The failure-to-pay penalty rate increases to a full 1 percent per month for any tax that remains unpaid the day after a demand for immediate payment is issued, or 10 days after notice of intent to levy certain assets is issued.
For taxpayers who filed on time, the failure-to-pay penalty rate is reduced to ¼ of 1 percent (0.25 percent) per month during any month in which the taxpayer has a valid installment agreement in force.
Combined penalties: For any month both the penalty for filing late and the penalty for paying late apply, the penalty for filing late is reduced by the penalty for paying late for that month, unless the minimum penalty for filing late is charged.