Vinyasa Flow Yoga is a term that covers a broad range of yoga classes. This style is called Flow, because of the smooth way that the poses run together and become like a dance. In Vinyasa Flow, the movements are synchronized with breath and the breath acts as an anchor to the movement as you flow from one pose to the next in time with an inhale or an exhale.
Vinyasa Flow allows for a lot of variety as it may make use of aspects of any number of yoga traditions. This style will almost always include sun salutations and you can definitely expect movement, not just stretching. Whether the class is fast or slow, includes inversions (shoulder-stand, head-stand etc), or is very alignment-oriented will depend on the individual teacher and the styles in which he or she is trained. Some classes include some warm up exercises at the beginning while others start with standing poses. The one thing you can be sure of is the flow between poses. The rest is up to the teacher :)