Restricted Stock Units (RSUs) are a form of compensation offered by some employers. These are generally taxed at the time of vesting, whereas employee stock options are usually taxed at the time of option exercise. The employer is required to withhold taxes as soon as the RSUs become vested.

The employer will issue Form W-2 to report the amount of income and withholding. See Form W-2 - Entering in Program for details.

In some cases, you will also receive Form 1099-B. See Form 1099-B - Entering Capital Gains and Losses in Program for details.

You may need to talk with your employer or plan administrator to determine what exactly happened and how much is being reported on Form W-2 and Form 1099-B.

Also see Publication 525.