Per IRS Publication 15, it will be taxed as "supplemental income" (like a bonus) and, yes, withheld at a higher rate. That is separate from wages (look at your last paycheck with a bonus to see they are different line items).
In theory it is withheld at a higher rate, but your overall tax rate is not changed, so your have a higher chance of getting money back.
In practice, that isn't usually the case after the crackdown on itemized deductions and changes to withholding. Effectively you are right in saying "taxed at a higher rate", but just wanted to explain how the IRS justifies this to be (hopefully) helpful.