From publisher blurb:
An enlightened royal diplomat and a silver tongued crime lord's advisor may not seem to have much in common. Beyond their associations and colorful presentations, however, there are many shared traits. Quite often these two both come from a lesser NPC class known as the linguist.
As expected, linguists are masters of language. These individuals are scholars of the spoken and written tongues as well as experts on non-verbal forms of communication, such as symbolic gestures and body language. A linguist is also capable of relaying simple communication with those that do not share a common tongue through the art of "pidgin". These characters also have a deal of insight into human nature, allowing them unusual core class skill options such as Forgery and Perform.
Though certainly not of suitable adventurer stock, linguists are still very useful and often highly regarded by explorers for their talents. Its not terribly rare to see a linguist with a few levels under their belts hanging around with a group of traveling adventurers or serving a thieves guild master as a translator and diplomat.
The Linguist is an extension of the non-adventurer worthy NPC classes, designed with the 3.5 SRD rules set.