Pega Web Mashup enables you to embed a Pega 7 application in another web application. You can create a simple mashup to display information in a harness or embed a case so that users can create work, get the next assignment, or open an assignment.This PDN page contains detailed description of Mashup: Pega Web Mashup
On the other hand USD is an application that allows Dynamics CRM to interact with other applications.
It brings a unified interface to all the disparate apps, and a common way to call them together. For example, let’s say you wanted your telephony system to be able to pull relevant customer information from CRM. Most customer support and call-centered phone systems have some kind of API that lets you “talk” to the system, and receive data from it when a call comes in. Now when that call comes, the USD could pick up the call, recognize the verification information your phone system required to get to support, and load the CRM screen with your customer’s information on it, ready to work. But that’s not just it. Say your CRM only contains your relationship information, but not your transactional information. You could also use that customer ID entered in during the initial phone call to pop the last bill of your customer from your transactional system right beside the CRM information.
But I am not sure about Pega - USD integration feature.