Q. What is the difference between EndNote 21 / EndNote Web / EndNote Basic and EndNote Online? 

EndNote 21 is the desktop application you can find on university computers via the software centre or download onto your own computer. It's the 'main' version of EndNote and the one we recommend you use. EndNote 20 and 19 are also still supported by Clarivate. 

EndNote Web is Clarivate's newest internet based application, only available to EndNote 21 users. Use this if you want to use EndNote with Google Docs and synch your references between devices. Use this in preference to EndNote Basic or Online. 

Endnote basic if available free to all. EndNote Online is an upgraded version of EndNote Basic, available to Web of Science users. These two web based systems are fairly old and have less functionality than the other versions. 

More information available here.