The 1040 FORM is an Internal Revenue Service (IRS) FORM used to report an individual's income, deductions, and credits for a given tax year. It is the most commonly used FORM for filing taxes in the United States. The 1040 FORM is also known as the 'long FORM' because it is the most comprehensive of the IRS forms. It is used by individuals who have more complex tax situations, such as those with multiple sources of income, deductions, or credits.