I've talked about WPS as a stack and as a Russian doll: WPS is built on WESB which is built on WAS.
But what functionality is in WPS? That's a long discussion, 'cause WPS contains a whole lotta stuff. Where to begin?
WPS is a merger of three previous products:
- WBI Server Foundation v5.1 (WBI-SF) -- Built on WAS, adds Process Choreographer for running BPEL workflows
- WebSphere InterChange Server v4.3 (ICS) -- Runs in Java, integrates business activities and events between applications, has some workflow-like functionality
- WebSphere MQ Workflow (MQWF) -- Adds workflow capabilities to WebSphere MQ's messaging capabilities; workflow and messaging are a very common combo
So the common theme here is workflow. WPS, as much as anything else, is unifying WebSphere's different workflow products, taking the best features of each, combining them with BPEL standardization, to form a best-in-breed workflow product.
This also means that if you use any of those three products, you should start thinking about migrating to WPS. If you're starting significant development of a new application, you should look into whether you'll be better off in the long run developing for WPS instead of one of these other products.
Here's one article that you may find helpful: "Migrating WebSphere Business Integration Server Foundation V5.1.x projects to WebSphere Process Server V6"