The IRS can take most - but not all - of your wages if you owe for back taxes and have not paid them. Unlike other forms of wage garnishment, the employer does not have to get your permission first, and is liable to the IRS for amounts paid to you instead of the amount that was supposed to be applied to the tax levy.
The amount withheld is determined based on the number of your dependents and the standard deduction to which you are entitled. When your employer receives notice of the wage levy, you should be given a copy of the notice immediately. On the back of the levy notice is an exemption claim form, where you designate the number of dependents and deductions. This form must be returned to the IRS within three days of when you receive it. If you do not return the form, the law authorizes your employer to pay you only $116 per week, and remit all of the rest to the IRS, until the taxes are paid in full or the collection period (ten years from when the taxes were assessed) expires.
If you owe state and/or local taxes, your wages may also be garnished by those agencies, which may act even more quickly than the IRS to seize back taxes. State laws vary, and may have different limits as to what percentage of your income may be withheld.