Most people automatically pay taxes through withholding from their paychecks. If, however, you receive taxable income from which there is no withholding, you are still responsible for paying taxes on it to the Commonwealth on a regular basis. The quarterly tax payments you must make in these situations are called estimated taxes.

Taxpayers are generally required to pay at least 80% of their annual income tax liability before the year's return is filed. This requirement is met through withholding and by making estimated tax payments on any income that is not subject to withholding. Estimated Tax continued  

Estimated Tax Resources
Should You Be Paying Individual Estimated Income Taxes?
Register to Pay Estimated Tax with WebFile for Income
Guide to Personal Income Tax