Treasury Stock are the shares of the company that it keeps in its own treasury.
These shares may have been bought back from shareholders or never sold in the first place. Because these shares are owned by the company itself, they receive no dividends, do not have voting rights and are not counted in Shares Outstanding.
Companies maintain treasury stock for a few reasons. The shares may be kept after an Initial Public Offering (IPO) or other offering to maintain the possibility of easily generating cash by selling the stock. Companies will also maintain treasury stock to help prevent a hostile takeover by owning a portion of the company's shares.