what is witholding Tax?

Withholding tax is the tax amount that your employer deducts from your paycheck. This term is usually used to refer to the federal income tax withheld. When you file your return at the end of the year, if too much has been withheld you'll get the extra back as a refund; if not enough has been withheld, you'll have to pay the difference.

Withholding tax is usually an amount deducted from income in country A before the net amount is paid over to a resident of country B. Among other things, it aims to stop people/companies avoiding tax by not declaring overseas income since it has already been taxed at source.

Edit - My answer is based on UK experience, I am guessing you guys are in the States and the terminology is used differently there?!

Withholding tax is the income tax withheld from your paycheck and used to operate the government. A certain amount goes into your Social Security account for your retirement or disability.