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This tutorial teaches you about the Atlassian plugin development platform. 
This tutorial introduces you to tools and APIs you can use to develop plugins for any Atlassian application. 
You'll create a plugin of your own that works in any Atlassian application. 
Your plugin will use AUI (Atlassian User Interface) templates for its look and GUI, and be able to store and retrieve data. 

You might notice that Atlassian often uses the terms 'add-on' and 'plugin' in tools and documentation. 
A plugin is a type of add-on you can develop for use in Atlassian host applications. 
A plugin is distinctive in that it runs in the same VM as the Atlassian host application.

Full documentation is always available at:

https://developer.atlassian.com/x/jIP5