The violin is usually called a "fiddle" when played in folk music styles. The instrument is the same, just played in a different style, so the fiddle is not a different instrument from the violin. Fiddle music from around the world includes Irish, Bluegrass, Scottish, Nordic, Klezmer, Romanian, Canadian, Western Swing, and many, many more.
Folk music is normally identified with a geographical region or a culture. It is traditional music that is rarely written, but instead passed down orally. Each performer makes the old melody his or her own through adding personal touches and changes. Usually the original composer is unknown.
Learning about a culture or one's heritage can't be complete without some knowledge of the folk music of that culture. The violin or fiddle plays a major role in many folk music styles, so its a great instrument to use for exploring various people and places in the world.