See this article:
http://word.mvps.org/faqs/formatting/WorkWithSections.htm

A section break must be inserted whenever you want to change any of the
section properties. These include (but are not limited to) page size and
orientation, margins (including header and footer margins), header and
footer, columns, page numbering, and page border.

In most cases, a Next Page break is required. To change the number of
columns, a Continuous break suffices.

In long documents, a section break is often used at the beginning of each
chapter in order to take advantage of the "Different first page" feature.

"Special formatting" for a section is limited to the section properties
mentioned above; other properties, such as default font, style definitions,
and certain document-level settings, apply to the entire document.