RSU are taxed when delivered, the vesting date also called exercise date. The value of the RSU at the time of vesting is considered taxable employment income and subject to any state and federal income and employment taxes. The income and withholdings should be included in the W-2 for the year of vesting.
Offered to employees as part of compensation, RSU- type of compensation in which each RSU- represent the right of the employee to take ownership of stock on a preset future date.