An agent is a member of your organization who uses your helpdesk, and has an account that lets them access the agent interface to view user tickets, edit help content, etc.
Agents aren't necessarily full-time support staff. Your managers, administrators and business analysts may need agent accounts.
You can control which of your agents are allowed to view the admin and reports interfaces. There are also extensive permission controls for the agent interface so you can control exactly what agents are allowed to do.
DeskPRO licensing is based on the number of agent accounts within the system. Agents cannot share logins. There's no limit to the number of users you can help.