Difference between ActionURL v/s RenderURL in JSR286 Portal

The difference is,
I would say:

ActionURL - forwards your request to processAction()
RenderURL - forwards your request to doView()

The most important difference is how the portal must handle both requests.
As the portlet specification says:

If the client request is triggered by an action URL, the portal/portlet-container must first trigger the action request by invoking the processAction method of the targeted portlet.

The portal/portlet-container must wait until the action request finishes.Then, the portal/portlet-container must trigger the render request by invoking the render method for all the portlets in the portal page with the possible exception of portlets for which their content is being cached.

The render requests may be executed sequentially or in parallel without any guaranteed order.