An agent desktop is a required component of an IPCC deployment. From the agent desktop, the agent performs agent state control (login, logout, ready, not ready, and wrap-up) and call control (answer, release, hold, retrieve, make call, transfer, and conference).
Within the Cisco Intelligent Contact Management (ICM) configuration, an IPCC agent desktop is not statically associated with any specific agent or IP Phone extension. Agents and IP Phone extensions (device targets) must be configured within the ICM configuration, and both are associated with a specific Cisco CallManager cluster. When logging in from an agent desktop, the agent is presented with a dialog box that prompts for agent ID, password (optional, depending upon agent configuration in the ICM), and the IPCC phone extension to be used for this login session. It is at login time that the agent ID, IP Phone extension (device target), and agent desktop IP address are all dynamically associated. The association is released upon agent logout. This mechanism enables an agent to hot-desk from one agent desktop to another. It also provides for laptop roaming so that an agent can take their laptop to any IP Phone and log in from that IP Phone (assuming the IP Phone has been configured in the ICM and in Cisco CallManager to be used in an IPCC deployment). Agents can also log in to other IP Phones using the extension mobility feature.
This URL should help you:
http://www.cisco.com/en/US/products/sw/custcosw/ps427/prod_technical_reference_list.html