A Tarantella is a very fast dance in 6/8 time. The name comes from the town of Taranto in Italy. People used to believe that if someone was bitten by a tarantula spider they needed to do this dance in order to drive out the poison. The dancer would get more and more excited until he (or she) seemed mad.
Some people think that maybe these people never really believed this, but that it was an excuse to dance at a time when the church said that dancing was not allowed. The spider is, in fact, only a little bit poisonous.
Some composers enjoyed writing fast pieces in 6/8 time which they called "Tarantella". Mendelssohn wrote one as the last movement of his Symphony no 4 (the Italian). Chopin and Liszt also wrote Tarantellas.