We have discussed how to start building add-ons that extend JIRA's functionalities. The add-ons are integrated very closely with JIRA's existing features; however, there are times when you need to add a few functionalities on top of JIRA so that other tools can interact with JIRA. JIRA provides access to various operations via REST.
Issue operations such as create/modify/delete issues
Group management operations like adding/removing users from a group
There are a lot of resources that are available through REST API. You can get the detailed list of all the resources at https://docs.atlassian.com/jira/REST/latest/.