Accessory Buses are buses that offer power and/or control of accessory devices that are attached to them. There are actually several type of accessory buses ranging from simple AC all the way to DCC based. Each type offers advantages and disadvantages DEPENDING on what your specific goals are.
1) AC Accessory Bus.
This bus is typically between 12 and 16VAC.
What devices consume AC power.?
1) Low Power "Twin coil" switch machines such as offered by Atlas and Roco were the most common AC accessories used.
2) Static incandescent light bulbs used for building interior and/or exterior lights (street lights).
3) Action animation buildings such as culvert or cargo loaders found in advanced toy train sets.
4) Action animation cars such as log or ballast car that unload themselves found in advanced toy trains sets.
History: This is the oldest type of accessory power used since it could be provided for inexpensively as an extra feature on a given train set "Power Pack" throttles. All they needed to add was a set of terminals for the AC power. Internally it was just a bunch of wires since the AC transformer was inside the PowerPack. Later almost all advanced commercial DC Throttles also had a set of AC terminals that provided accessory AC power.
High Power: On large layouts or layout with lots of accessory loads, these same commercial throttle's "AC terminals" did not offer enough power (current) to run all the AC accessories connected to it. The solution was buy an inexpensive open frame transformer plus appropriate fuse/circuit breaker to protect it and wire it up. Although the wiring is pretty simple, it does require someone who knows what they are doing. There we safety issues in doing this. TODAY one purchases self contained AC power supplies that are pre-wired up internally with protection so that all you need to do is plug it in to the AC outlet and connect the low voltage AC wire to the AC Accessory Bus.