Apostille is certificate which verifies and confirms the seal and signature of person who authenticated the document . Some people call this 'superlegalisation'. There are designated authorities in every country to issue apostille certificates or legalisation. Apostilles are commonly issued for various documents related to adoption cases, for commercial documents, for official documents related to vital statistics, and for court records, land records, school documents and patent applications.

Not all countries can issue apostille, but only the countries who had signed 'hague agreement for abolishing the need for public documents'. Once apostille is obtained, it is automatically considered 'valid' and 'acceptable' in all the 98 hague convention countries. Some small territories or notably some countries, very well recognise apostille, eventhough they cannot issue apostille. See: Hague Convention Website. If the country where the document will be used is not a party to the Convention, you will have to begin the cumbersome, time-consuming process of obtaining a series of certifications known as the "legalisation".

Documents issued from one country, cannot be simply considered valid in another foreign country. Even sometimes a documents authenticated by public notaries cannot be accepted. you will be asked to obtain 'apostille' or 'legalisation', only then it will be considered valid. Ex. Powers of attorney, affidavits, birth, death and marriages records, incorporation papers, deeds, patent applications, home studies and other legal papers.

The following are considered as public deeds within the meaning of that Hague Convention:

• Any document issued by an authority or by an official covered by a jurisdiction of the State, including those coming from the public prosecutor, a clerk of the court or a bailiff;
• Administrative documents such as deeds from the public records office;
• Notarial deeds;
• Official declarations such as references of registration, visas for a set date and certifications of signature, applied to private agreements.
  

* No reference in the list alongside the name of the country: the ordinary legalisation procedure applies

Links

http://www.hcch.e-vision.nl
Apostille Discussion Forums
http://travel.state.gov/authentication.html