A security is in bearer form if possession of the security confers ownership. No register of ownership is maintained.
Ordinary shares are not usually bearer securities and the main record of ownership is not the certificates that are issued, but the company's register of shareholders. Certain types of bonds are commonly issued as bearer bonds, the most important being Eurobonds. Bearer shares and ADRs exist, but bearer shares are a rarity.
There are obvious security problems associated with the handling of bearer securities. Some types of bearer securities (including Eurobonds) are traded through systems that minimise physical movements of the securities by depositing quantities of the bearer security securely in banks. The registered ownership of the deposited securities can then be traded.
Bearer securities are also regarded, by governments, as making tax evasion and money laundering easier. This has lead to some types of bearer security becoming rarer.