Technically, a grant of restricted stock units is an unfunded promise to issue a specific number of shares (or a cash payment) at a future time once vesting conditions have been satisfied. Stock-settled RSUs are much more common than cash-settled RSUs (which are subject to unpopular variable accounting), and they are used widely enough to rival the use of standard restricted stock
You can think of RSUs as a restricted stock grant in which the stock itself is not issued or outstanding until both vesting and the release of the shares. Generally, restricted stock units are subject to the same tax rules as restricted stock once the shares are received. To further compare the differences, see the sections Restricted Stock and Restricted Stock Units.