You use a bus when you only want a certain percentage of the single to be routed through it. For example Parallel compression or if you want to frequency split a bass. The possibilities are pretty much endless.
Almost. With a group in Ableton for instance, you can "group" things together, then add effects to the entire group. However, you cannot send various amounts of each element to the FX within that group. This is where Bus channels differ, also known as send tracks, from here you can have say a delay on a send / bus channel, and vary the amount of dry/wet that goes into it from each element.
For me grouping is essential to keeping things neat and tidy, I can group all my drum channels to one, bass to another etc etc, I can compress them together to help them gel, add reverb for space, and from here I can send varying amounts of each separate element, to a send channel for parallel compression
I use Fx channels to send effects to a whole group if needed or to individual tracks (in Cubase 6.5) doing this also give me the ability to adjust the wet/dry for each part I send it to. So if I can do this no need to worry about bus channels?
I don't know how Cubase works so cant really comment on that section. If you can group, and effect the elements within that group at varying amounts then I guess theres no need to bus?! But how would you parallel compress?
You answered your own question with 'varying amounts' I think? assuming there's a dry/wet on the compressor anyway..
They're essentially the same thing, a way of summing multiple audio signals
DAWs all have their own system, if you look at an actual mixer it makes more sense, you have the aux sends, usually with pre/post fade and send level controls, you can patch in inserts if you want to send the whole signal to some external processing and you have the sub-groups that you can use to bus a sub mix (drums etc)
BUS is just a signal path to route audio (1 or more inputs) to a specified output, which is what a group does in Ableton, but in logic you can control the amount being sent to the bus, which makes it an aux send as well so it varies between DAWs
The real difference is between busses (2 or more inputs at output stage of signal) and inserts (1 input at input stage)