Characteristics
"Lingua franca" is a functionally defined term, independent of the linguistic history or structure of the language: though pidgins and creoles often function as lingua francas, many such languages are neither pidgins nor creoles.
Whereas a vernacular language is used as a native language in a single speaker community, a lingua franca goes beyond the boundaries of its original community, and is used as a second language for communication between groups. For example, English is a vernacular in the United Kingdom, but is used as a vehicular language (i.e., a lingua franca) in the Philippines.
International auxiliary languages such as Esperanto have historically had such a low level of adoption and use that they can only be described as potential rather than functioning lingua francas.
Read more about this topic: Lingua Franca