You define process variables and then, in a task node, you receive parameters and put results. You map these parameters and results to process variables. So you share information between nodes with the process variables.
In the work item handler, you retrieve the parameters from workItem.getParameters(), and you put the results in the way you mentioned (.completeWorkItem).
If you are in a gateway, you can access directly to a process variable by its name. (you can use it in a constraint for example).
Regards,
Demian