JIRA Service Desk is a wonderful add-on for JIRA, that allows you to create service portals for your customers, deflect requests by linking the customer portal with a Knowledge Base, manage SLAs and so much more. JIRA Service Desk introduces a new licensing model, and this posts explains how Agents work in that new paradigm, as well as the impact on your license.
Image credit: Atlassian
In "plain Vanilla" JIRA, you have users and that's it. Of course, you can setup Project Roles and Groups, but still, users remain users. In the JIRA Service Desk world, we have three classes of citizens: Agents, Collaborators and Customers.
In JIRA Service Desk, Agents are the users that work on customer requests and communicate with customers. That means Agents can access the Customer Portal and the actual service desk interface in JIRA, they can see both. That means they have access to queues, reports and SLA metrics for the service desks they have access to (because you can setup several Service Desks!)
By default, Agents can access and edit requests in the service desks they are assigned to. While working on support requests, they can add comments and choose if a comment is customer-facing or private (internal). Naturally, they can also edit and delete comments.
More importantly, Agents are the only ones who can transition a request.
JIRA Service Desk Agents may also manage content in the knowledge base, provided you've linked JIRA Service Desk with a Confluence Knowledge Base.
An Agent consumes one JIRA Service Desk license AND one JIRA license.