Restricted stock is stock of a company that is not fully transferable until certain conditions have been met. Restricted stock is often used as a form of employee compensation, in which case it typically becomes transferrable (“vests”) upon the satisfaction of certain conditions, such as continued employment for a period of time or the achievement of particular product-development milestones, earnings per share goals or other financial targets. Restricted stock is a popular alternative to stock options, particularly for executives, due to favorable accounting rules and income tax treatment.
Restricted stock units (RSUs) have more recently become popular among venture companies as a hybrid of stock options and restricted stock. RSUs involve a promise by the employer to grant restricted stock at a specified point in the future, with the general intention of delaying the recognition of income to the employee while maintaining the advantageous accounting treatment of restricted stock.
For more information, see https://en.wikipedia.org/wiki/Restricted_stock