the short answer is that transfers are only between DVC members and transfer ownership of the actual pts. the new owner of the transferred pts can use them (based on those pts' home resort and UY, not the transferree's home resort and UY) to make a reservation that they control. the DVC owner who transferred the pts no longer has any risk related to the use of the pts.
only one transfer is allowed per UY, either in or out (not both), so they can be tougher to get. DVC contracts forbid accepting compensation for a transfer.
rentals are reservations made and still owned by a DVC member which can be rented to members or nonmembers (but the renter only owns the reservation after check-in.) if a renter trashes the room or manages to leave credit card charges unpaid, DVC may follow up with the DVC member.
these threads may explain it further, if you are curious:
One negative of transfer ins are that unless it is the same resort and UY as your contract you will not be able to see the points or make reservations on line. You also can not borrow transferred in points