There are two categories of docs: Guides and API docs. Guides explain general topics and use cases (e.g. setup). API docs are automatically generated from the codebase and give specific details about functions, properties, and events.
(Corrections and additions welcome)
Setup - The setup documentation gives a deeper view of the additional methods you can use to trigger the player setup.
Options - There are a number of options that can be used to change how the player behaves, starting with the HTML5 media options like autoplay and preload, and expanding to Video.js specific options.
Tracks - Text Tracks are used to display subtitles and captions, and add a menu for navigating between chapters in a video.
Skins - You can change the look of the player across playback technologies just by editing a CSS file. The skins documentation gives you a intro to how the HTML and CSS of the default skin is put together.
Tech - A 'playback technology' is the term we're using to represent HTML5 video, Flash, and other video plugins, as well as other players like the YouTube player. Basically anything that has a unique API to audio or video. Additional playback technologies can be added relatively easily.
Plugins - You can package up interesting Video.js customizations and reuse them elsewhere. Find out how to build your own plugin or use one created by someone else.
Glossary - Some helpful definitions.
Removing Players - Helpful for using VideoJS in single page apps.