Print Current W4 Form (download)
The purpose is to submit this to your employer so that the employer can withhold the correct amount of federal income tax from your pay.
This way you can help to avoid getting a surprise payment due when you have to file your taxes.
Print Form W-4V (download)
Voluntary Withholding Request for Unemployment Compensation and Certain Federal Government Payments.
For the Voluntary Withholding, there are specific percentages that can be withheld - 10% 15% and 25% depending on the situation.
If you are exempt, read Publication 505 Tax Withholding and Estimated Tax (download)