There are four different form types available when setting up the basic details of your Form.
Main
This is the form that enables an item to be created, once you have set up your entire workflow and it is published the main form is first entry for that item. The main form is compulsory in a workflow or sub workflow.
Status
This form is optional. This is a form that appears at a given status or statuses. If for example at a certain status in a workflow you require a certain piece of information to be filled out then in that case you could opt for the status form.
Approval
This form is to approve a particular status and move it forward in the workflow. For example when you sign off on an item to move it forward in a workflow.
Rejection
This form is to approve a particular status and move it backwards in the workflow.