What is the difference between
an apostille and an authentication?
Not much really.
Apostille and authentications are issued from the same federal or state secretary of state offices and both apostille and authentication certificates are printed out on 8 1/2 x 11 sheets of paper and then attached to your documents with a staple (TXSOS) and with a grommet (USDOS).
The difference is an apostille certificate is issued when a document will be presented (filed and sent) to a country that is a member of The Hague Convention.
An authentication certificate is issued when a document will be presented (filed and sent) to a country that is not a member of The Hague Convention.
For example, all documents being presented and sent to China would receive state and federal authentication certificates and for Mexico, you would receive state or federal apostille certificates.