ICT is an umbrella term including all technologies regarding information and communication as e.g. Internet, multi media, etc.

Interoperability means the ability of ICT systems and of the business processes they support to exchange data, use the exchanged data and to enable the sharing of information and knowledge.
With a more technical focus, it means the capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of those units

Source(s):
DEMO_net. Interoperability requirements and standards in eParticipation. Deliverable, DEMO_net Consortium, to appear in 2008.
European Commission. European Interoperability Framework for pan-European eGovernment services. Brochure, European Commission, 2004.