Each section in OneNote is a .one file in the My Notebook folder in the My Documents folder on your hard drive.
Sections are the basic files of OneNote. Each section is an OneNote (.one) file. So a section called Test is a file, Test.one.
If you use Microsoft Office Word, you're probably familiar with each document being a .doc file. Just as each Word document can contain many pages, each OneNote section can contain many pages. In OneNote, however, many sections are open at the same time, unlike Word documents.
Each OneNote section (.one file) is in the My Notebook folder in the My Documents folder on your hard drive.