A Russian doll (or Matryoshka doll) is a doll which opens to contain another doll and so on. In this way, one big doll actually contains many progressively smaller ones.
I've already talked about The WebSphere Process Server Stack, how WPS is built on top of WESB which is built on top of WAS. To be even more specific, they're built on top of WAS ND, which in turn is built on top of WAS Base.
Another way to look at this is as what we call the "Russian doll" architecture, shown here (again, my picture, not Marketing's):
The WebSphere Process Server v6.0.1 Contains Relationship
This is really just a different way of drawing the stack from before:
The WebSphere Process Server v6.0.1 Stack
What the Russian doll perspective emphasizes is that WPS has WESB built in, which it turn has WAS built in. WAS isn't just a predecessor that evolved into WESB and WPS; WAS is actually contained in them. They're running on WAS, so to speak.
This is significant for IBM because it means that we're not reinventing software servers from the ground up; we're building WPS on the proven WAS foundation. What's significant for you as a WebSphere customer is that all of your skills in administering WAS and deploying applications to it are still relevant and useful for WPS and WESB. Many WebSphere customers have invested significantly in building well designed WAS server farms; rather than needing to replace these, they can use these and build onto these to add WPS and WESB servers as well.
So WPS and WESB provide lots of new functionality, but they build on what you already know (and hopefully love) about WAS.