When a security is issued, it can take one of two forms - either registered or bearer. When in registered form, the corporation acts essentially as the transfer agent - that is, they know who has the security. If you sell it, they are notified and then keep a record of the new owner, how to reach them, how to beg them to vote YES on board resolution 19, etc.
Most securities today are issued in registered form - governments want to track who owns what in what companies. Shares of stock and store tremendous amounts of wealth as well - so evil-doers of the world have loved bearer form securities because they could hide the wealth in creative ways before unleashing their next attacks.
