What is the difference between nets and buses in Altium Designer?
A net is one connection, or rather one node, of a circuit. Anything on that net is connected to everything else on that net. A bus is a variety of different nets that are grouped together to clean up schematics and make them easier to read. Instead of having individual wires tracing all over the schematic, you can use buses to group them together and they appear as one thick wire.

You can think of a net as a single wire, and a bus as a bunch of wires bundled together.