Withholding is the amount that employers subtract from their employees' gross pay for a variety of taxes and benefits, including Social Security and Medicare taxes, federal and state income taxes, health insurance premiums, retirement savings, education savings, or flexible spending plan contributions, union dues, or prepaid transportation.
Contributions to tax-deferred savings plans are withheld from your pretax income, as are amounts you put into tax-free flexible spending and prepaid transportation accounts. Those amounts reduce the taxable salary that your employer reports to the IRS.