Microsoft OneNote is a very popular application. Not only can you note and save your thoughts there but you can also use it as a solid app for collaborative work. As it is possible to integrate it with the most popular project management and task tracking tools, this format became well-spread in the software development sphere.
A project in OneNote is called notebook. It is saved as a folder with data files for each section(page) of the notebook. These files have .one or .onenote extensions. As it is mentioned in Wikipedia, such a document can contain various types of data like pictures, text, taken by a stylus handwriting text, audio or video records, etc.
For those who don't have the application but have to work with OneNote files, here is a kit of solutions in here. These applications are free tools to convert, view, lock, or unlock Microsoft Note files online on Mac OS, Linux, Android, iOs, and anywhere without installing any additional software.
If you liked the offered by apps functionality and want to manipulate OneNote data programmatically please check the Documentation chapter.