Welcome to the workforce.
The W-2 is the form on which your employer has reported both your income for last year as well as the taxes you've already paid. This form gets sent both to you and to the IRS. You will use the information on this form to complete your tax return.
You generally get sent 3 or 4 copies of this form: one is for you to keep for your records, one gets sent in with your federal tax return (assuming you don't e-file), and one gets sent in with your state tax return, if needed.
There are instructions on the back of the form that roughly explain what the numbers in the different boxes represent. When you do your taxes, the instructions for the tax form tell you what to do with the numbers in some of these boxes.
If you use software to do your taxes, you will enter the entire form into the software: you'll be presented with a W-2 form on the screen, and you enter in the information just as it is shown. Then the software does its magic and figures out what to do with each value.
Here is a quick explanation of some of the boxes:
Box 1: This is the total taxable income for the year. Anything that you paid with before tax money (such as 401k contributions or health insurance premiums) has been taken out.
Box 2: This is how much in federal tax you have already had taken out of your paychecks.
Box 3: Some of your income is not taxable for federal income tax, but is taxable for Social Security. This larger amount is shown here.
Box 4: Shows the amount of Social Security tax you've already paid.
Boxes 5 & 6: Same as boxes 3 & 4, but for the Medicare tax.
Boxes 16 & 17 show the taxable income and tax paid for your state. If you are lucky enough to live in a place where there is a local income tax, you'll see amounts in boxes 18-20.
The amount you have contributed to your 401k is not always shown on your W-2. Luckily, you don't really need this number to do your tax return. Your Box 1 income number already has your 401k contributions taken out.