Difference between a bus and a wire

A wire can be a bus if it is a serial link carrying many individual pieces of information. More usually, a bus is regarded as a collection of wires that transport digital information from A to B. 64 bit processors (PCs etc.) have a 64 bit-wide bus between the CPU and their memory chips and possibly to other devices.

It doesn't have to be inside a computer of course - anything that is transmitting information from A to B will use some form of wire or collection of wires for achieving those aims.

What differentiates a wire as not being a bus is that it only carries one coherent "entity" such as power or a microphone signal or is connected to an on/off switch or a guitar or a speaker. A bus is usually digital.

I usually consider a bus a union of many wires. Imagine an address data bus with A15..A0 .. 16 wires, 1 bus. This is valid for very low level hardware.

Once you think about protocols, a bus is usually more a description of a topology type.

One differentiating feature of a bus is that more than one device on a bus can send information. A device on a bus not only receives information; it can also reply. If it replies over some different wire(s) than the one(s) where it receives, then both (sets of) wires make up the bus.

If the information comes from a single source, and all the other devices are simply passive listeners with no way to reply, that's not a bus.