Hey,
A buss is used for 'submixing', or putting a series of channels under the control of one fader.
Imagine this:
You have a mixer with 16 inputs, all of them are being used. Let's say that on channels 1-4 you have a snare, kick, hats and other percussion; channels 5-11 you have an assortment of sounds like your main sample riff, strings, guitars, other sounds, etc., and channels 12-16 you have a bass and some vocals. Everything is panned, leveled and eq'd the way you like it.
However, after listening to your mix you think that the drums are good, but the strings (channels 5-11) are too loud and the bass/vocals (channels 12-16) aren't loud enough. You COULD grab faders 5-11 and pull them down and faders 12-16 to pull them up, but it gets a little funky if the faders aren't all level because you have to make sure that your fingers are steady...
However, you could assign channels 5-11 to buss 1/2 and channels 12-16 to buss 3/4...you would then simply move the buss 1/2 faders down and the buss 3/4 faders up, and all the volume, pan and eq settings would follow properly. And you're only having to use 4 fingers instead of 12! Sometimes a buss will be a stereo pair under the control of a single fader, this will mean that you would only need 2 fingers to control them.
That's just one of many uses for busses, I'm sure that there are many more...
The amount of busses available on a mixer is a selling point because it gives the user more control over the mix. When buying a mixer, it will usually tell you how many channels and busses it has in the model number...my 1402 has 14 channels and 2 busses, a 1202 has 12 channels and 2 busses....a 1604 has 16 channels and 4 busses etc..
Take care,
Nick