Office 365 (OneNote) - What is OneNote?
Microsoft OneNote is a note-taking app that syncs across all of your devices. OneNote can accomplish a great variety of tasks such as: bulleted lists, keeping screenshots, documenting important parts of emails, etc.
OneNote "Notebooks" are organized in the Notebooks button, the sections of those notebooks are in farthest left column, and pages within those notebooks are in the column beside that.
The + Section button can be used to create a new section for notebooks, and + Page can be used to create pages within those sections. The Home tab on the ribbon opens a menu similar that of Microsoft Word for aid in typing/formatting notes.
NOTE: The OneNote app is available on a multitude of devices and syncs your latest changes to your notebooks.