What's the difference between a process and activity variable?

Hello,

I'm trying to write a Bean Shell script that reads the value of a workflow variable. However I see that it is possible to read in a 'process variable' and an 'activity variable'. There only seems to be a getter for process variables by using the following method:

WorkflowManager.getProcessVariable()

For activity variables the only getter I can find is:

WorkflowManager.getActivityVariableList()

However there are setters for both process and activity variables:

WorkflowManager.processVariable()

and

WorkflowManager.activityVariable()

My question is this: What is the difference between a process variable and an activity variable? And if I only want to read 1 activity variable, do I really need to get the entire list first?

Thanks!