Documents certified by notaries are sealed with the notary's seal or stamp and are recorded by the notary in a register (also called a "protocol") maintained and permanently kept by him or her. These are known as "notarial acts".
apostille or legalisation
In countries subscribing to the Hague Convention Abolishing the Requirement for Legalization for Foreign Public Documents only one further act of certification is required, known as an apostille, and is issued by a government department (usually the Foreign Affairs Department or similar). For other countries an "authentication" or "legalization" must be issued by the Foreign Affairs Ministry of the country from which the document is being sent or the Embassy, Consulate-General, or High Commission of the country to which it is being sent.
In Hong Kong, this is done by a notary public (around 400 in Hong Kong) through the High Court.
For more information on Apostille service, also see www.ApostilleSeal.com