An apostille is a certificate of authenticity issued by governments for documents to be used in another country.  It was established by the 1961 Hague Convention, and only applies to signatory countries, which the U.S. and UK, are, but Canada is not. Canadians, instead, may be required to have a Certificate of Authenticity, which is essentially the same thing by another name.

How to get an apostille in the U.S.

How to get an apostille in the UK

How to get Canadian Certificate of Authenticity