Either a park or DTD. That is the only way to travel between two resorts using Disney Transportation(unless of course you're traveling between resorts on the monorail or the resorts that use the boats in the Epcot area).
Is there a specific resort that you are planning to travel from and then another to travel to? If so, many of DISers could quickly answer that question. Or is this an inquiry in general?
It all depends on where you want to go and from where. Some resorts are easier to get to than others. Let us know where you need to get to, and from where, and we can give you the best routes.