Restricted stock is an outright grant of common stock subject to vesting and company repurchase rights for unvested shares. Used primarily for founders and very early employees in C-corp startups, the recipient owns the shares from grant date, can file an 83(b) election to lock in tax treatment at the near-zero grant-date value, and starts the long-term capital-gains holding clock immediately. It is structurally different from stock options (which are rights-to-buy, not ownership) and from RSUs (which are promises to deliver shares in the future).
The mechanic of a restricted stock award: