Vinyasa is a fast-paced series of postures or asanas that focuses on the flow between movements, rather than individual poses.
Vinyasa stands in opposition to hatha. Hatha classes tend to focus on one pose at a time with rest in between.
In contrast, flow classes string poses together to make a sequence. The sequence may be fixed, as in Ashtanga in which the poses are always done in the same order, but most of the time vinyasa teachers have the discretion to arrange the progression of poses in their own ways.