Lingua franca refers to any language used for communication between people who do not share a native language. It can refer to hybrid languages such as pidgins and creoles used for communication between language groups. It can also refer to languages which are native to one nation (often a colonial power) but used as a second language for communication between groups. Lingua Franca is a functional term, independent of any linguistic history or language structure.
Whereas a vernacular language is the native language of a specific geographical community, a lingua franca is used beyond the boundaries of its original community, for trade, religious, political or academic reasons. For example, English is a vernacular in the United Kingdom but is used as a lingua franca in the Philippines. Arabic, French, Mandarin Chinese, Spanish, Portuguese, Hindustani and Russian, serve a similar purpose as industrial/educational lingua francas, across regional and national boundaries.
International auxiliary languages created with the purpose of being lingua francas such as Esperanto and Lingua Franca Nova have not had a great degree of adoption globally so they cannot be described as global lingua francas.