This excerpt taken from the UPS DEF 14A filed Mar 13, 2009.
Restricted Stock Units
Restricted stock units are bookkeeping units, the value of which corresponds to one share of class A common stock. The Compensation Committee believes that restricted stock units provide a retention incentive and enhance executive stock ownership and shareholder linkage. Restricted stock units vest 20% per year over five years. Upon vesting of restricted stock units, the individual receives shares of UPS class A common stock. When dividends are paid on UPS common stock, an equivalent value is automatically credited to the participants bookkeeping account in additional restricted stock units.