After WWII, countries around the world came together to decide on a uniform document certification that they could all accept as official. The result was the Convention of October 5, 1961 under the formal name “Abolishing the Requirement of Legalization for Foreign Public Documents”. This convention brought about a basic simplification of the series of formalities which complicated the utilization of public documents outside of the countries from which they issued.
The Apostille certification basically certifies a document for international use. It certifies the document as valid to be used by other government agencies. It is issued for the specific document it is attached to and has a serial number.
You can read more about it on the official Hague Conference on Private International Law website.