Cisco Unified Contact Center Express.
Seats are the equivalent of Agent Licenses. 50 seats would mean that 50 concurrent agents would be able to login to the Desktop client (CAD or Finesse) at any point of time.
Premium Seats would mean that you have a Premium package on the system which would entitle you to multiple benefits (check the guide for a comparison between ENH and PRE for the full list) such as Chat and Email, Advanced Scripting steps, etc.
The beauty of the Premium seat is that 1 PRE seat = 2 IVR ports (limited to the capacity defined by the OVA). This would mean that your system would have 100 IVR ports defined.
Hope this helps!