Worker role and web role counterpart in GAE
yes there is. look at backend and frontend instances. your question is too broad to go into more detail. in general the backend type of instance is used for long running tasks but you could also do everyrhing in the frontend instance.
I am currently working with MS Azure. There I have a worker role and a web role. In worker role I start an infinite loop to process some data continously. The web role is performing the interaction with the client. There I use a MVC Framework, which on server side is written in C# and on client side in Javascript. 
Now I'm interested in GAE engine. I read a lot about the app engine. I want to build an application in Python. But I don't really understand the architecture. Is there a counterpart in the project structure like the worker and web role in Azure?