A flow step can be a form or one of the activities provided in the flow designer palette. You will see New Form, Summary, HTTP or Email activities that can be dragged and dropped from the paltette in the flow work area. Your existing forms can also be added as flow steps.
The Form Viewer control is used to allow a generated PDF to be viewed as part of a form activity in a flow. This control only appears on the Forms palette when you click the 
icon to edit an activity in the flow designer.