I've talked about the Origins of WPS. I've also talked about how WPS is built on WESB, which is built on WAS (Russian doll view, stack view). So what are the parts in WPS and WESB?
The model is shown pretty nicely in this picture from "WebSphere Process Server: IBM's new foundation for SOA":
WebSphere Process Server platform
The picture is still accurate but has changed a little with the introduction of WESB. Imagine a new layer or row, above SOA Core but below Supporting Services, called "mediations." Mediations were in WPS v6.0 somewhere but are now a distinct new feature in WESB v6.0.1 and therefore WPS v6.0.1.
So again, this picture shows that everything's built on WAS. With the imaginary Mediations layer, it also shows again the difference between WESB and WPS. WESB contains the Mediations and SOA Core layers (on top of WAS). WPS adds the Supporting Services and Service Components layers on top of WESB (and WAS).
So if you want the features in the Service Components and Supporting Services layers, you don't get them with WESB, you need WPS.
In future postings, I'll discuss what these individual components are.