EMan said:
Went to my first Klezmer concert just a few months ago. The town of St Augustine, Florida offers free concerts of all types throughout the year. A friend of mine, another accordionist, told me he was appearing in this Klezmer group at the concert. Interesting music. This particular group fronted a clarinetist as the main focus of the music.
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<SIZE size=125><COLOR color=#0040FF>The Clarinet will usually be the star in a Klezmer group. Im trying to change that... 
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Zevy is correct in that the clarinet is usually the lead instrument in a Klezmer group. Now, that wasnt always the case. The genre started out as Eastern European Jewish dance music. It was meant to accompany dances at special events, mostly weddings. However, two forces, one in Europe and the other in the US, brought the clarinet into prominence.
As Jewish (and other) musicians were drafted into the army in Eastern Europe, they had to learn to play more military instruments -- brasses and reeds. When they came out of the army, it was soon recognized that the clarinet had tremendous carrying power. It could be heard at much greater distances than an ensemble of strings.
In the US, as some of these musicians began to arrive, they came to the attention of the record companies. These record companies recognized early on that because immigrant groups would likely buy the music of their homelands, they began to create catalogs representing the various ethic music genres of the immigrant groups. Jewish music was no exception. And because clarinets recorded much better than strings using early acoustic recording methods, it soon became the lead instrument here.
Now, how do accordions figure into this music?
Here is a long and scholarly article about that:
http://www.budowitz.com/Budowitz/Essays_files/The Klezmer Accordion.PDF.zip
But if you dont want to read it all, you will find it summarized in this book:
http://www.press.uillinois.edu/books/catalog/86tsp7zm9780252037207.html
By the way, Josh Horowitz, the author quoted in both of the above links, is a fantastic teacher and musician in his own right. He plays a nineteenth-century, three-row chromatic button accordion as well as an instrument called a tsimbl, which is somewhat like a hammered dulcimer and is a traditional Eastern European Jewish instrument.
