Google Chrome Apps are web applications that run on the Google Chrome web browser. Chrome apps can be obtained from the Chrome Web Store. It is a "stand alone application or tool". You can find all your apps by clicking on the apps launcher as shown in the photo below on MacBooks. Students on Chromebooks can find it under the search (magnifying glass).
Extensions are small software programs that can modify and enhance (extend) the functionality of the Chrome browser. Extensions are also obtained from the Chrome Web Store. Once an extension is added, you will find them to the right of your "omnibox" - see photo below.
An Add-on will enhance the capabilities of Google Docs, Sheets, and Forms. These are added with a Google Doc, Sheet, or Form...not from the Chrome Web Store.
Here is a great Slides Presentation