What is Queue definition in Microsoft CRM?

Queues are places in organizing, prioritizing, and monitoring the progress of your working items in CRM.

In CRM following entities are enabled for Queues by default:
Appointment
Campaignactivity
CampaignResponse
Email
Fax
Incident
Letter
PhoneCall
RecurringAppointmentMaster
ServiceAppointment
Task


Reference:
http://msdn.microsoft.com/en-us/library/gg328459.aspx