To the parks:
WL and FW: boat to MK, bus everywhere else. Bus transportation from WL is OK, from FW it is not good as you need to take at least two busses and sometimes three just to get to a park.
BWI/BWV, YC/BC, Swan/Dolphin: bus to MK and AK (they share the same bus except for MK where Swan/Dolphin now has its own) and it is OK but can be long if you are last on the resort stops; boat or walk to Epcot or MGM; boat ride is pleasant, walk is usually faster.
All others: bus to all parks; each resort has its own bus to the parks but trip can sometimes be long as various stops can be made within the resort.
Among the resorts:
BWI/BWV, Swan/Dolphin, and YC/BC are all within short walking distance of each other and you can also take the boat (Again walking is usually faster).
There is a boat that travels the loop of FW, WL and Cont.WL and FW are also connected by a bus that also goes to the TTC and thus makes it easy to get to MK resorts particularly Poly.
It is possible to walk between OKW and PO and then PO and Disney Institute but it is lengthy.
There is a bus that runs between Coronado Springs and BW.
Other trips between any resorts require you to take two Disney transports, usually going to a park first and then switching to another transport at the park (you can also go to DD and make the switch but that trip takes a long time).