The Cha-cha-cha (in Spanish chachachá) is a Latin American dance and style of music derived from the rumba and mambo in 4/4 meter. In ballroom dancing it is increasingly popular to call the dance “cha-cha”.
The term "cha-cha" comes from Haiti, where it referred to a component of a bell which made a "cha-cha" noise when it was rubbed. The device was kept and used as an instrument.

The music of cha-cha-cha evolved from mambo. In the late 1940s, mambo was wildly popular across the United States, but it was a very fast and difficult dance. Orchestras slowed down the mambo, and cha--cha-cha was the result. In 1951, Cuban composer and violinist Enrique Jorrín introduced the cha-cha-cha rhythms under this name to Cuban dance floors while playing with Orquestra America. Some say that he came to this idea as early as in 1948 while being with Antonio Arcaño's orchestra. In 1953, his “La Engañadora” and “Silver Star” became recorded hits. The dance teacher Pierre Lavelle from the United Kingdom, a founder of the Latin American Faculty of the ISTD, visited Cuba in 1952 to discover mambo (some say, rumba) danced with the triple step in place of the slow one. He brought this dance idea to the Europe and eventually created what is known now as ballroom Cha-cha-cha.             ((mostly) from Wikipedia, the free encyclopedia )

The music for cha-cha is accented on the 1st beat and a percussive accent on beat 4. The cha-cha-chas are taken on beat 4 the 4a and 1. The slower steps are taken on beat 2 and 3. Consequently the dance is normally counted, “2, 3, 4 & 1”.  To begin on “1”, take a prep step to the side (the last beat of the previous chasse).