Form W-4 applies to jurisdiction United States of America
Form W-4 (officially, the "Employee's Withholding Allowance Certificate") is an Internal Revenue Service (IRS) tax form completed by an employee in the United States to indicate his or her tax situation (exemptions, status, etc.) to the employer. The W-4 form tells the employer the correct amount of federal tax to withhold from an employee's paycheck.Motivation
The W-4 is based on the idea of "allowances"; the more allowances claimed, the less money the employer withholds for tax purposes. The W-4 Form is usually not sent to the IRS; rather, the employer uses the form in order to calculate how much of an employee's salary is withheld. An employee may claim allowances for oneself, one's spouse, and any dependents, along with other miscellaneous reasons, such as being single with only one job. In the latter case, this creates an oddity in that the employee will have one more exemption on the W-4 than on the 1040 tax return. This is not a tax deduction in itself, but a procedure to prevent under-withholding for those who do not qualify. Nevertheless, it is legal to have "self" and "one job" allowances from the highest paid job, provided that all other jobs have zero allowances. 
Each year, the IRS issues tax refunds to tens of millions of Americans. In 2011 alone, the IRS issued refunds to more than 100 million Americans. That means three out of four returns filed for 2011 called for money back. All told, the government sent about $318 billion to taxpayers, with the average refund being around $2,900. Claiming allowances on the W-4 and updating it frequently effectively lets the taxpayer claim refunds ahead of time, by not overpaying in the first place. Over-withholding can occur if, for example, an employee receives a one-time bonus, or only a partial year is worked, as it may only take into account the current paycheck, rather than the year-to-date amount. (I.e., withholding is calculated as if the employee earned this amount every payday on an annual basis.)