Much like the OSI model Cisco has a standardized reference model to explain the various components that make up a unified communications system. From top down:
Endpoints – IP Phones, Cell Phones, Video Phone, IM Client aka what the user touches.
Applications – Voice Mail, Conference Call Apps, Call Center Apps, 911 Services aka feature rich services.
Call Processing – CUCM, CUCME, UC500 aka what handles call flow.
Infrastructure – ASA Firewall, Voice Router / Gateway, Voice switch aka what all the above layers need to communicate together.