Bus is just a shortening of busbar - a major conductor that other circuits run from. Bus is being used more and more these days and seems to be an American form - if you read Apollo 13, they gone on about the Main Bus frequently, Old World terminology would be Main Busbars, note the singular/plural difference.
A busbar can take many physical forms - http://en.wikipedia.org/wiki/Busbar - what is, or isn't, a busbar is dependant on the circuit form that's in use, it can be a bit of a matter of opinion, sometimes...
In general, the bigger it is, in cross-sectional area, the better, but the gain is very slight if you go beyond a reasonable size. It's just a matter of knowing what the maximum total current will be and having a suitable cross-sectional area to cope with that. In a static situation, it doesn't matter if it's solid, single core cable or multi-core flex.
If you are running high frequency signals down a busbar, then there could be some advantage to using multi-core flex - this is because the 'skin effect' will be reduced, but I would be surprised if it has a significant detrimental effect in a model railway scenario - I would be interested to know if anybody ever finds this to actually be the case.
There would, of course, be no harm* in using multi-core, and the extra cost would be marginal, so it might be worth doing anyway, to be on the safe side.
* apart from the need to secure it a little bit more often, maybe?