There is actually no difference, they mean the same thing (i.e. ticket in English), they are just used in different countries. Billete would often by used in the Castilian variant of Spanish spoken in Spain whereas Boleto would be used more often in Latin American Spanish.

A boleto bancrio, or boleto, is an official form of payment regulated by the Central Bank of Brazil that uses vouchers to pay. Instead of directly paying for a good or service online with cards or bank accounts, you can create a boleto to pay with cash, or more securely with a card.