Again another blog post that was requested by you the blogshere. As you all know the main OneNote file type is .one which is an individual section file and it lives, as all files in, in a directory/folder. You can open up folders as notebooks in OneNote and any subfolders will be Section Groups in OneNote. However there are a few other OneNote files that are important and in most cases you will never see, but if you are reading this blog then you like to learn about stuff so let me share with you all.
OneNote Single File Package - .onepkg
OneNote Packages are like zip files that allow you to share OneNote files across computers including all of the embedded files that were in those sections/files. Additionally you can send entire notebooks which you couldn't do before in OneNote 2003. If you are in OneNote and want to send an entire notebook to someone you can choose File-->Save as and select to save the entire notebook as a .onepkg see here:
Then you can send that file as an attachment and the recipient can open that file directly and it will take them to OneNote and show them the entire notebook. It is a great way to share files with people and you can't use a shared notebook. It is also a great way to archive all of your files. In reality this is just a CAB file so you can open the file yourself. Note: in earlier builds of OneNote 2007, like Beta1, these were called OneNote Archives and were .onea files (just in case anyone ever googles for ".onea" I wanted to have this out in the world somewhere)
OneNote Table of Contents - .onetoc & .onetoc2
Another file type most people have never seen before are the OneNote TOC files. These files are system files by default so unless you have told Windows to show them you probably haven't seen them. These files contain the ordering of sections within a notebook. The .onetoc was for OneNote 2003 and contain information about which files are open/closed and in what order the sections appear. In OneNote 2007 we have .onetoc2 files which has information about ordering of sections. If you delete this file then OneNote will open the notebook in alphabetical order and then you can reorder them yourself.
If you have questions please let me know.