A form can be composed of more than one section, and each section can have an entirely different layout to other sections on the same form (but they all have the same page size, as determined by the form size. Sections lend themselves to, for example, printing an invoice with a followup up page of terms and conditions. They are probably best avoided for things like cheques, where special stationery is required.
When printing a multi-section form, all sections will be output together (i.e collated). Similarly when PDFing, all sections will be output to the same PDF file. This differs from form groups, which are collated separately, since they may use different stationery.