Jazz dance is a form of dance commonly used in Broadway shows and movies. Jazz is a less serious looking, looser kind of dance compared to ballet, for instance. Even though jazz dancing might look easy and fun when the dancers do it, most times the dancers have to be in really good shape, and practice sometimes six hours a day. Some traditional jazz numbers are 'All That Jazz' and 'Chicago'.
In jazz dancing the movements are big and exaggerated and there is usually an attitude the dancer conveys to the audience. The attitude would depend on the dance. For example in a modern number like "Livin' La Vida Loca", the dancer would probably be happy, and look like they were at a party having a really rockin' time. That would give the dance a profesional look. Jazz dancing is also used in modern dancing as on MTV. Las Vegas showgirls are also jazz dancers.
Just about every dance school teaches jazz.