Difference between Approver, Role Approver and Org Approvers

Question

Can someone briefly describe the difference/purpose of the Role and Org approver capabilities. Specifically as if I set someone up as Just a role approver then I can't see them in the Available Approvers list for a Role setup anyhow.
As a follow on to this is there any way to not allow an approver responsible for an organization also see and approve approvals for a role's approver for a role that is in the organization. It seems to destroy the point of having role approvals if the org approver can still simply execute his approvals form them.
Z

Answer 1

Ideally Roles were established to provided access controlls. You can route any work items to any "Approver" based on this role. It can done in your workflows.
To answer your question, in simple terms, Role's can be spanned across organizations. So, any request made to have a designated " Role", then owner (or Role approver) can approve this request irrespective of the organization he belongs to. Ex: You can your AD Admin for your AD account who is in IS organization.
Similarly for Org approvers. These approvers basically confirms the user, org CRUD operations.
I hope this helps.