JIRA Plugin Development utilizes a platform of Atlassian ecosystem, Atlassian Plugin SDK. This SDK allows to develop add-ons for Atlassian products with ease.
This software development kit (SDK) of Atlassian encapsulates a several years experience based on user cases, engineering solutions and business workflows to provide a set of ultimate tools to solve different problems as may be required by a customer or a developer. What JIRA, Confluence, Bitbucket and other Atlassian products can do is to enable one to write awesome and flawless code in a matter of using the SDK and the API to code.
Add-ons created with Atlassian SDK add one or more modules to an Atlassian products like Jira or Confluence to deliver desired useful functionality to the user. For example, an add-on can provide a new type of Custom Field for JIRA. Or, one can use a workflow validator to validate the inputs when the status of a Jira issue is being changed. An add-on can provide the user with a new type of Jira Gadget for the dashboard, add Web Items and whole sections to menus at different levels, provide remote access via REST resources and a lot of other useful stuff.
Atlassian developers team has worked hard to provide a customer or a developer with cookbooks and manuals on how to develop, test and troubleshoot software. This support helps one to keep moving with their plugin development and getting a clear reference on what steps to take next in the process.