Estimated Tax Payments: How to Determine What You Pay and When

How can I figure out if I must pay estimated taxes, and, if so, how much will I owe?

if you're an employee, your employer withholds taxes from every paycheck and sends the money to the IRS and possibly to your state government as well. This way you pay your income taxes as you go. And, if you're like most wage earners, you get a nice refund at tax time.
But if you are self employed, or if you have income other than your salary, you may need to pay estimated taxes each quarter to square your tax bill with Uncle Sam. You may owe estimated taxes if you receive income that isn't subject to withholding, such as:

Interest income
Dividends
Gains from sales of stock or other assets
Earnings from a business
Alimony

More info at : http://turbotax.intuit.com/tax-tools/determining_estimated_tax_payments/small-biz-article