You've heard people talk about buses, auxes, sends and returns before, but what's the difference between all of them? They all have to do with routing audio through your DAW, and this article will clear up any confusion between them. These terms stem from a time before DAWs. Everything was analog and routed manually through massive mixing consoles.
Each vertical row of knobs and faders is a single channel strip with controls that you would typically find in the mixer section of your DAW (volume, pan, solo, mute, etc.). These mixing consoles look big and scary, but they're really just numerous channel strips lined up next to each other. In order to perform group processing, you have to route the output of your tracks through a bus. In order to perform parallel processing, you have to send signal to an auxiliary (aux) track. This holds true in all the DAWs that I've come across as well. They're all modelled after mixing consoles, so the way signal is routed through them is similar to how it's routed through a console.
A bus is a point in a signal flow where multiple channels are routed into the same output. Different instrument groups are often bussed and processed together to make them sound cohesive. Songs will typically have a drum bus, instrument bus, and vocal bus. The master channel in your DAW is also a bus and is commonly referred to as the master bus. It's where all of your track outputs merge together before leaving your DAW.
2. Aux Tracks
An auxiliary (aux) track accepts signal being sent to it from various different tracks; the main thing that differentiates it from a regular track is that you can't record audio to it. The signal that gets sent to an aux track is a duplicate of the original. The benefit of using an aux track is that you can apply processing to a duplicate signal without affecting the original track's signal. This gives you control over the mix between the dry (original track) and wet (aux track) signals. Buses group tracks together (allowing for group processing) and auxes affect the duplicate signals of tracks (allowing for parallel processing).
If you ever hear someone use the term "return track," they're referring to an aux track. In Ableton, aux tracks are called return tracks and it's acceptable to use the two terms interchangeably.
A send is a knob or fader within your DAW that allows you to send varying amounts of a regular track's signal to an aux track. While the terms "aux" and "return" can be used interchangeably within one another, a send is something different. It's simply a parameter that can be adjusted.
If you have any questions about routing audio within your DAW, leave a comment below! I'd be more than happy to help you out.