Originally simply one of several Hinayana schools, Theravada and Hinayana have now almost become synonyms. This is not so much due to a misunderstanding, but has come about because Theravada is today's only surviving school of the Hinayana tradition of Buddhism.
Today, the major countries where the Theravada school is dominant are Myanmar, or Burma (since the 11th century), Kampuchea (since the 13th century), Sri Lanka (since 250 BCE) and Thailand (since the 13th century).