Trace (as you know) is just the material on the board through which conducts electricity. Bus is a collection of physical connections, in addition to specifying how it works, i.e. a bus might be 64-bit, and transmit on both rising and falling edges of the clock signal. Or it might be 8-bit and use strobe instead of clock for syncing communications. Bus is logical in that these properties are independent of the existence of traces; you could built a 64-bit DDR bus or a 8-bit strobe bus out of wire or even pipes with water in them if you wanted to (although from engineering point of view neither is practical of course); even though "bus" generally implies the type of physical connection, the two can be separate.
Or, perhaps a poor analogy, but here goes: "trace" is like a piece of conducting wire (come to think of it, that's exactly what it is). All it is good for is transmitting electricity. The wire doesn't care what kind of electricity it transmits or what it's for; it might connect a battery to a device, it might be used to transmit Morse code using 1-, 5- or 12-volt DC (the revolutionary Morse bus specifies the lengths of tones and clicks as well as the voltages that may be used, but doesn't care how the sender and receiver are connected), or it might carry the data signal of USB. Perhaps I should use USB as an example: USB is a bus, and specifies the voltages used and how data is transmitted using the cable. A USB cable is, in a way, a "trace": it really doesn't care what kind of electricity goes through it, it simply sits there allowing current to flow between the devices so that they can communicate.
