'Sets' are created by YOU using Polyvore's features, clicking and dragging each picture onto a 'Set' in a layered fashion. Here is a Polyvore that I did for fun below. It didn't take long and Polyvore helps with your creation by providing lists of pictures in a grouping by color!
Art and Expression is HUGE on Polyvore. The community forms groups for you to join or you can create your own groups and start collecting fans of your work.
Here is an Expression I did in Polyvore:
Check out Polyvore for yourself. It's fun!