In simple words, an Apostille is a certificate that is attached to public documents. 
The Apostille is the result of a simplified legalization process that has been developed by some countries that, in 1961, have adhered to a treaty called the Hague Convention. This has made it easier for an institution to deal with foreign public documents.

The actual certificate is a piece of paper that is normally added to the back of a document. However, the Apostille does not have a standard design, therefore, this can also be an ink stamp or an adhesive sticker.

Although the design may vary, the content of an Apostille should include important information that cannot be omitted such as ISSUING COUNTRY, DATE, and THE ISSUING OFFICER’S SIGNATURE.