Unified Service Desk for Microsoft Dynamics 365 Customer Engagement provides a configurable framework for quickly building applications for call centers, so that agents can get a unified view of the customer data stored in Dynamics 365 for Customer Engagement apps. This means that USD is a desktop application that can be highly customized, and allows customers to configure the location of their components, listening for and emitting events that can trigger different actions or other features.
See also: USD 4.1 Overview