Thanks AT.  I'm going to open a dummy project tomorrow and just mess with something like a test guitar track using that dry and reverb mix you mentioned as an example. I guess where i'm getting confused is here.  You see i can understand that things like reverb use a lot of CPU so you dont want to put a reverb on three different guitar tracks. So you make a bus and put the effect on the bus.  But what's losing me is why you send a dry signal to one bus (using the OUTPUT option) and then from the SAME track send a signal to a different bus that has an effect on it using the SEND option.
If I might use an analogy here it seems to me like you are playing your guitar through two different amps.  One amp is playing a clean sound and the other has a reverb and they both ultimately go to some master output speaker.  So I'm wondering why bother doing this.   If you want reverb, get rid of the amp playing a clean sound.  Am i correct in my analogy there or have i lost the plot? 

Think about why you are using the reverb. It defines the space around the instrument.
Yes, ultimately, everything should end up at your master bus, but by isolating the space and the instrument(using the send to one bus and the track output to another), you can control each independently. You can move the instrument around in the space, such as pulling it forward for more definition.
That may not make sense at first, but play with it for a while.
This may help:
http://forum.cakewalk.com/Working-with-Buses-m1302521.aspx
It's an old thread, based on an old product, but the idea is the same. Hopefully the interface doesn't throw you off.

Just so you also understand the basics. 
 
For groups of instruments we use sub mix buses  which then output to the Master bus.  
There are also the EFFECTS  buses
the Master buss
the Metronome is also a buss. 
With some audio interfaces we can also have Headphone or Cue Mix buses. 
 
You can insert as many stereo buses as you want in Sonar, they are all the same until you assign their outputs. 
 
There is also two main ways to send your tracks to these buses. 
The track OUTPUT and the SEND. 
Most tracks will default to the Master as the output. 
You'll use a sub mix buss to group instruments. 
We use the SEND to route the track to an Effects buss, It is in parallel using the effects sends to send "part" of the tracks output to that bus for processing. The tracks output ( Dry)  will still be pointed at the Master or a sub mix buss. 
 
You sounded confused about this part so hopefully this explains it a bit. 
 
  "SEND"  will show all available buses but you will normally only send  to an effects bus from here. The other use is to a headphone or cue mix. Note that sending to the effects bus can increase the overall volume of that track because there are now 2 audio pathways to the master. 
 
You might have already figured this out but your example only mentions one guitar going to a buss. Normally we only use a sub group buss for  2 or more instruments or a drum kit. 
 
As example, we generally only have one bass track. That will go directly to the master. Sending it to a "bass" buss is a waste of time. All processing can be done within the original Bass tracks efxs bin. 
 
But other instruments, like guitar, we might have 4 or 20 guitar tracks, This is where you send them to a sub mix bus. That way you can globally process them. Hope that make the concept more clear. 
 
The main function of sub mix buses is to group similar sounds so we can simplify the mixing process. You don't really ever need a sub mix buss. If your like me I don't mind each part going directly to the master because my songs never have more than 16 or 20 tracks. If I  had a higher track count I would defiantly use buses. I do use buses for when there are 2 guitars doing the same thing. I also will use a Drum buss on occasion. 

I just recently started using buses, thanks to this forum. For what it is worth, I am fairly new to using Sonar in the first place, at least on the level at which I use it now.
 
My drummer has 10 mics for his kit, 2 of which are overhead and he is not done yet.
 
For the kit as it stands now, I use a buss for the overheads and a buss for the remaining drums.
 
This is very useful in mixing and leveling the overall output and feel of the kit.
 
I also use FX buses. I still use FX bins for individual tracks, but sometimes to get what I am looking for, I use a buss.
 
I am sure that is because I am not that good at what I am doing yet.
 
If I need to have more than one type of delay effect (in different parts of the lead) on the lead guitar, I will end up using a buss for the second delay effect.
 
I create all of these envelopes for the first delay effect and then when I try to use the the same process for the second effect, I gets all convoluted and messed up and I am sure this has to do with how I am write enabling, which that in itself requires much work to get correct, but I digress. So, instead I just create a buss and it's done.
 
I just finished a few tracks using all of the above, and it is essentially my first time getting it close to correct.
 
I love doing it and have spent enormous amounts of time editing these tracks to get what was just a "jam" in a room where everyone shared the same four non-acoustically treated walls, to a decent sounding song.
 
EQ, levels, transients, FX, gains, fade in/out, track editing etc. etc. I actually took the original tracks and exported them just so that I could A/B them with my finished work, so that I could judge my own efforts. I can say that there is no comparison to those originals or the old way of how I did things before I used busses.
 
 
 
 
 

I still think in terms of using a mixing desk.
The Bins are like the Channel inserts.  So your Channel (track)  has to pass 100% of it's signal through this in/out pathway
Then the Effect buses are like using the Aux sends of the mixer and in parallel. They can be set to pre and post just like a mixer too.  
 
So to me the bins are for EQ and compression. 
The sends for reverbs and delays.

Hi RobertB.  OK, i see what you're saying about being able to move the sound around using the dry and effects sounds.  This is something i'll just have to go and play with.  I'm looking forward to testing a track with a dry sound plus a send to an effects bus.  I think this would be useful especially if using reverb as i find reverb can be overdone and washes the sound out.  So perhaps a little dry sound in there could help. 
Hi Cactus, just to clarify, i was only using one guitar track in my example just to keep it simple.  I too would not be using a bus if i only had one track in a real world scenario.  Such as bass as you mentioned yourself. 
Thanks for all the replies guys.  lots of good links in some of those replies and yes it's true the screenshots are from earlier versions of Sonar but i'm guessing the theory of it is the same. 

Incidentally, "Sends and Buses" would make a great electronic music album title.