Withholding tax, generally speaking, is income tax withheld from an employees' wages and paid directly to the government (either the IRS or a state or local government revenue department) by the employer, and the amount withheld is a credit against the income taxes the employee must pay during the year.
Taxes may also be withheld against certain other types of income, including pensions, bonuses, commissions, and gambling winnings.
The IRS has a very informative guide to tax withholding at the following link:
https://www.irs.gov/individuals/employees/tax-withholding