What is Form W-4?
The W-4 is the short form you hand your employer to control how much federal tax comes out of your pay. It's the steering wheel for your withholding.
How it works
The current W-4 skips old-style "allowances" and instead asks about your filing status, other jobs, dependents, and any extra amount you want withheld. Payroll plugs your answers into IRS tables to set the per-paycheck figure. You can update it any time — it doesn't go to the IRS, just to your employer.
A quick example
What people get wrong
Revisit your W-4 after any big change: marriage, divorce, a second job, or a working spouse. Two earners who each fill it out as if they're the only income are the classic reason couples end up owing.