What's the difference between a common clustered system and a Petri net?

Maybe I'm comparing potatoes with apples. I'm working with YAWL, a workflow language that is based on Petri net.

I'm at the very start of beginning to understand it, but there's a point that I doesn't catch: what is the difference of a "classic" clustered system - or a cloud or grid system - and a Petri net?

I mean, for example, what are the differences of a clustered system of a Django webapp, for example, and a webapp written in YAWL?

Edit

I found this business process management engine, Activiti, integrable in Spring Boot. What are the differences between a webapp written in YAWL and a webapp written in Spring Boot + Activiti?

As mentioned in a comment, Petri nets are abstract mathematical models. They have nothing to do with clusters or webapps, unless you want to model a cluster or webapp as a Petri net.

YAWL and the tools around it seem to implement a workflow engine, with some interfaces to webapps. That means you can define workflows in a graphical environment, and somehow bring them into the web without writing a lot of code. It's a domain-specific solution.

Django is a general web framework. You can do all kinds of stuff with it, not just workflows. But you have to write the code for whatever you want to do.