Circuit Training is mixture of resistant and aerobic training, that is usually broken into timed intervals. An example of a work out might consist of 12 stations working out a specific area for 30 seconds, then resting in between each station for 12 seconds (Stations can consist of sprints, burpees, pull-ups, etc.). Did I mention that is only considered one round, and there can be anywhere from two-five!
Circuit Training can be found in a variety of places, anywhere from Bootcamps, to customized gym, or even that TRX machine made ever so popular from its television ads. It was developed in England by Morgan and Anderson in 1952 as a time efficient way to become healthier and stronger. One of the downsides to circuit training, is it unrealistic to build muscle in bulk. That reason though is also why alot of people choose to circuit train, it allows them to lift without looking like the hulk.
One of the most recognizable forms of circuit training is P90x. It's fast growing popularity led to hundreds of machines and videos that allowed individuals to bring circuit training into their homes. Ads proclaiming "In just 10 minutes a day, you can look like this!" Although some of the ads may have been misleading, it has been proven that a proper circuit training workout will deliver results.
Bobby a circuit trainer from Results Only announced to me "Don't worry, your allowed to curse in this gym and if you don't want to, your doing something wrong." Which follows suit with my description of circuit training, 30 seconds of hell that delivers an amazing work out.
What happens when you get someone who loves aerobic exercises, and dance? Zumba happens. In 1986 a man named Perez started Zumba as result of forgetting his normal work out music tapes to an aerobic class. After some improvisation, Zumba quickly spread into the United States in 2001. Zumba is not just dancing, it can incorporate squats, lunges, and even resistants training! It can be done to slow or fast songs, however it is usually a mixture of both. Zumba has found its place in several markets including studios, videos and now even video games! Its fan base has even grown to celebrities who regularly zumba including Madona and Jennifer Lopez. Since it is labeled as a fun, and great work out; Zumba has taken hold and growing fast in the fitness world. 
