Bookmarks allow you to save the address of a web site for quick access later.
Extensions, allow you to "hack" the Chrome browser (like a mod in a game) to add useful features or keep your browser free of things you don't want.
Apps run inside the browser, like apps you install on your tablet or phone.
Apps give access to web-based applications (eg. Google Drive/Docs/Slides, Classroom, WeVideo, Soundtrap, YouTube). To access your apps you can use the App button on the Bookmark bar.
Within the Google Apps (Docs, Slides, Sheets,..) Add-Ons add extra capabilities or features like bibliography organisers, mathematical expressions, graphing tools, and formatting tools.