Don't confuse restricted securities and restricted stock. They are very different.
Restricted stock, along with its nearly identical twin restricted stock units (RSUs), is an outright grant of company stock to employees or other service providers. The stock is "restricted" because it is subject to a vesting schedule, which can be based on time or performance goals, and is governed by other limits on transfers or sales that the company can impose.
Before dilution concerns and changes in stock option accounting made restricted stock and RSUs more attractive to companies, these awards were usually granted only to key employees and executives. Many companies now grant restricted stock and RSUs more broadly to employees and managers either in place of stock options or in combination with them.
Restricted securities are shares that are not registered with the SEC, such as shares in a privately held company. They are resold under SEC Rule 144 or another registration exemption. The restrictions in restricted securities come from the securities laws, whereas the restrictions in restricted stock come from your company. When the shares are owned by a senior executive or director (i.e. an affiliate), they are also called control stock, even when acquired from an open-market purchase or from stock compensation. Technically, the phrase "restricted shares" also refers to restricted securities, though some companies use "restricted shares" to mean restricted stock grants.
When you acquire restricted securities, the stock certificate will have a legend stamp indicating that the shares are restricted and therefore cannot be resold unless they are registered with the SEC or unless an exemption applies. As explained in another FAQ, this legend will need to be removed before you can resell the stock.