What are the differences between these workflows in terms of how
sharepoint handles them.

From my understanding,
The VS ones are created by user as a solution with code and deployed
as a dll. The info like mapping between Doclib - WF is stored in the
database. Also, once the WF starts, all state info is tracked in the
DB. Is this right?

What about the SPD workflows? Does SPD also generate code and dll and
deploy it to the server? or is the whole content and config of the wf
stored in DB? How does this wf maintain state? How can I change this
WF without using SPD?

Does VS or SPD support BPEL?

Also, another related question.. Can I user aspx forms in VS workflow
instead of IP forms?