The word klezmer (derived from the Hebrew "vessel for music") originally referred to a musician, but in the 20th century came to refer instead to the musical style. The music arose in medieval eastern European Jewish communities to perform at simkhas (holidays and celebratory events, especially weddings). It's origins lay in Jewish folk music and cantorial chanting, with elements of other local ethnic folk tunes (especially those of the Roma) and military band music. When brought to the United States by Jewish immigrants in the 20th century, it was further influenced by jazz and theatre music.
In addition to eastern European-derived pieces, the modern klezmer repertoire is typically augmented by Yiddish melodies, Israeli folk music, tunes from early 20th-century American Yiddish theatre productions, and modern compositions in the traditional style. Klez Project's performances span this full range of the repertoire.