Yoga Flow are classes which usually have a faster pace and are intended to be more athletic and challenging. Poses are often connected with sun salutation movements, often using music to set the pace and flow of class.
Flow and Vinyasa evolved from Ashtanga Yoga. In the original Ashtanga Yoga tradition, classes were not held on Saturdays, but since Saturday is a popular yoga day in the West, Ashtanga teachers like Tim Miller and Larry Schultz created sequences out of Ashtanga Yoga postures to teach for fun on Saturdays. Those sequences which are called "Sun Salutation C" and "Rocket Yoga" evolved through their students into Power Yoga, Vinyasa Flow and numerous other styles of yoga. They all teach the same hatha yoga poses, but in different orders and different paces.