Restricted stock units (RSUs) are a promise to grant shares of stock to an employee, either on a vesting schedule or when the employee reaches certain milestones with the company.
When you receive an RSU award, you don't actually own the stock until it vests. Accordingly, there is nothing to report at the time of the award.
Once the stock has vested, the fair market value of the stock gets reported as ordinary income, usually in box 1 of your W-2. In some companies, employees can earn dividends from unvested RSUs—these are also reported in box 1 of their W-2 forms.
After vesting, you own the stock outright. Should you later sell those shares, you'll get a 1099-B, which will report the gain or loss from the sale.