Form 1040 is longest and allows all deductions and credits. The Form 1040A is shorter and simpler but several deductions and credits are not available for 1040A filers. The Form 1040EZ is the shortest and simplest form, but only allows for the standard deduction and few credits. Which form you should file depends upon how many if any credits you are eligible for and whether you can itemize your deductions. Read each form carefully and use the simplest form that allows you all of the deductions you are entitled.
Many people misunderstand the difference between a deduction and a credit.
A deduction reduces the amount of your taxable income. This means that you do not pay taxes on income equal to the amount of the deduction. A deduction only reduces your tax liability by the percentage of your marginal tax rate. In contrast, a credit is a direct reduction to a taxpayer's tax liability. The IRS provides every taxpayer with a standard deduction of several thousand dollars.