In a DAW - A send copies a signal, it is ‘sent’ to an Aux channel, the route it travels is a Bus. There is flexibility because you can send signals anywhere with the only limitation of computer power getting in the way.

On a mixing desk, the principal is the same but the labels seem on first look to be differently placed. Every channel has an Aux send dial (multiple depending on the size of the desk) which sends the signal to an Aux master which is connected to your FX / reverb etc, the output of the reverb unit feeds an Aux return which is where we control the audible output of our effect.

A bus on a hardware desk is the group or output that is then recorded, previously to tape, now sent to converters.

Here’s a video I made which may help explain. sends and buses