HelpDesk for JIRA is a simple user interface for the most popular user actions: create, view, edit and comment issues. All HelpDesk's interfaces are friendly and familiar because they are looked like common Jira interfaces.
The plugin provides the best way to do support for thousands external or internal users because they don't need a Jira login permission (like jira-users members). So, you should have to Jira license just for your support team (aka agents). There is a polite license policy for support needs. Let's see the User's Guide to learn basic functions.
Also, look more about differences between HelpDesk for Jira and Service Desk for Jira.