jkoseattle
OK. So I know there's a forum for this kind of post, but everyone has been so helpful on getting me from 8.5 to X3, lots of help about Take Lanes, ins and outs, and assorted other things, I just thought I'd share the song I've been working on all this time on this forum. Since this is going on the album I'm pulling it off SoundCloud in a couple days, and this is just a first mix. 
 
https://soundcloud.com/jkoseattle/we-are-the-elders-v1/s-O6Qvq
 

Great song, well done! It has a fine authentic vintage sound which reminds me a little of the Mothers Of Invention...ear candy!

Difference:
 
Routing the output of a track to a bus means your entire signal is going to the bus.
 
Using a  Send means the output of your track is going two places: the bus (which is the main output) and the send (which is a secondary output often used for effects).
 
With a Send, your main signal keeps on going to the destination, but a part of it splits off and goes down a side road to meet up with the main (original) version later.  Imagine a single road: that's the bus option.  Now imagine a side road splitting off the main road and merging back in later.  That side road is the Send.  Your choice is whether to put the effect in the main road so everything gets doused with it, or put the effect on the side road so you can determine how much gets the effect.  You can send 10%, 40%, or however much you want down that side road.
 
By using a Send, you can use the Send control to turn the reverb amount up or down without affecting the main signal.  With a bus, you can't just turn the reverb up and down: you have to turn the whole signal up and down.
 
Not a super scientific explanation but maybe it will help.
 
Note: In many studios send are used for a lot of things, including setting up separate headphone mixes and other things; but I think in Sonar, effects like reverb would be the main use.
 
Short bit of advice:  Always keep a dry (no reverb) signal available whenever possible.  If all you have is the version with the effect, you may regret it later.  You never know when you may want a different effect, a little more dry signal mixed in with the effect version, etc.  When you are mixing, options are good.
 
In your example, if you really just want a special effect, you can put the effect in the bus, just understanding that you will have less control.  I might do that if it were a sound effect or something.  I would not do it for vocals or main instruments because of the lack of control everyone mentioned.

I kind of did things backwards. I set the output of my vocal track to my VoiceLive 2 unit, which is how it is supposed to be used, as an insert.
 
Then I added a send to the main mix to add a little clarity. It also adds a slight delay that ain't bad either.