Lingua Franca, also known as Sabin, was a pidgin language, meaning it was a language composed of the simplest words and grammatical structures of two or more existing languages. Pidgin languages tend to evolve when groups of people with widely differing languages find themselves in a position where communication would be greatly to their advantage, and that was certainly the case with Lingua Franca.
Lingua Franca was a language of trade. It grew from the need to describe merchandise, to name prices, to count, haggle, and form commercial arrangements and relationships. As such, it was spread by the traders themselves and by the sailors who necessarily accompanied them, and it was added to by those with whom they traded and interacted.
The language consisted mainly of words from Italian, Spanish, Occitan (a variety of French spoken on the Mediterranean coast), Greek, and Arabic. In addition, there were regional variations which contained words native to specific locales. As with all pidgins, it employed the very simplest rules and structures. For example, verbs were only used in the infinitive, so there were no tenses to learn. Additionally, the vocabulary was small and very directly related to the primary purpose of the language.
Lingua Franca was in use from the Levant (an area which comprises the eastern Mediterranean and the Aegean from Greece to Egypt and includes Israel, Jordan, Lebanon, the Palestinian Territory, and the western half of Syria) to the Iberian peninsula (which comprises Portugal and Spain). The first record of its use is from the fourteenth century. It survived up right up until the nineteenth century, at which point it completely disappeared, most likely due to the increasingly common and widespread use of standard French.
In its heyday, it was undoubtedly a dynamic and quickly evolving language, subject to additions and alterations as necessary in order to best serve its purpose. One source describes Lingua Franca as "Mediterranean Esperanto", which is certainly figuratively accurate. Of course, Lingua Franca was actually widely and usefully employed, and Esperanto never has been.
Because of its very specific purpose, Lingua Franca existed as a spoken language only, and the fragments of it that survive are almost exclusively contained in the writings of travelers who recorded snippets of conversations conducted in it. Aside from those snippets and some secondary sources which appear to contain traces of it, it has all but vanished.. Its one very obvious legacy is that its name, which literally means 'Frankish Language' in Italian, is now used to mean 'something approaching a common language'.
Sources:
- "Extract from lecture on Lingua Franca" by Alan Corré at http://www.uwm.edu/People/corre/franca/edition3/lingua2.3.html
- "Introduction to the Study of the Lingua Franca" by Charles Häberl at
http://www.people.fas.harvard.edu/~haberl/franca.html
The page at http://www.uwm.edu/People/corre/franca/index.html contains a short glossary of words used in Lingua Franca