Just get yourself a map of WDW. Easy. No need for bus routes to be enumerated on any map. It's all very simple.
From every resort, you can get on one single bus to get to any single theme park, Downtown Disney and water park. That is, if you are leaving from your resort, you never need to transfer to a second bus to get to a park or DTD.
So if you are at Pop and want to go to Epcot, just go to the Pop bus stop (at the resort) and wait for a bus that says "EPCOT" on the front display. It will go to Epcot. If you at Pop and want to go to Magic Kingdom, just go to the Pop bus stop (at the resort) and wait for a bus that says "MAGIC KINGDOM" on the front display. Couldn't be easier, right?
Getting from one resort to another resort can involve a little more finesse to do it as efficiently as possible. But the general rule others have told you applies quite nicely -- take a bus from Pop to a park or DTD, and then from that park/DTD catch a bus to the other resort. You can look at the "bird's eye view" map of DTD to determine the most efficient park to use as a bus transfer point. For example, if you are at Pop and want to get dinner at Animal Kingdom Lodge, you would NOT want to take a bus from Pop to MK and then a bus from MK to AKL, as that would be very far out of the way. Looking at a map, it makes a lot more sense to get a bus from Pop to Epcot, MGM or AK, and from there catch a bus to AKL. Looking at a map of WDW, you'll see that Epcot, MGM and AK are more or less on the way between Pop and AKL, and that means it would be more efficient to use one of those as a bus transfer point rather than MK or DTD.
One other variable to keep in mind is making sure your intended bus transfer point will still be open when you want to use it. When all else fails, DTD is consistently open late all year round, even during slow times when the parks close pretty early.
There's the monorail that, resort-wise, serves only Contemporary, Polynesian and Grand Floridian. And there are a few boats here and there. But that's an advanced class ... try digesting the basics first!
