First a few definitions:
CCA is 2 things..
User Interface Integration for Dynamics CRM (UII) is the product code that we provide as a Composition framework for building agent desktop applications for Dynamics CRM. We don't ship the source code for this, Just Samples of controls
and how to do things.
Customer Care Accelerator (CCA) (which is where the CCA name comes from) is the Sample code / agent desktop we provides that shows a technical implementation of UII. We ship a sample agent desktop you can use a the basis for your
development.
Now...
Unified Service Desk (USD) is a full supported Agent Desktop built on UII. USD includes a large number of configuration first features that allow you to configure most of what of what you had to code in custom desktops built on UII.
For example, the look / feel / layout are now all style sheet and XAML layout driven vs code driven. We still provide a highly extensible coding model based on UII with a number of enhancements we support in specifically in USD. To get started
with USD you should take a look at the
Admin guide, and the linked Dev Guide.
CCA continues to exist as a sample on UII and is now located in the CRM SDK Downloads.
Hope that helps,
Mattb.