This a copy of Mr. Spartypants from a yahoo post that I think fits real well for the subject
Every motorcycle design (or design of anything!) is a compromise between different goals. Engineers know you get something up to get something else, and the trick is balancing the different things you want. Years ago, nearly all bikes were 'standards'. If you wanted a touring bike you'd put a windshield or fairing on the bike, and bags. If you wanted a sportbike you'd put on stickier tires, stiffer suspension, low handlebars and a little bullet fairing. But the last few years bikes have been designed from the ground up for specific purposes.
A sport bike compromises everything to speed and handling. Modern sportbikes are really almost like racing bikes with license plates. So they are not comfortable. They have an extreme riding position that puts a lot of stress on the lower back and wrists, plus the seat is very small and hard because you get a lot of road feedback through your butt. They're popular mostly with young people because older people can't take that position for a long time. And they're not for beginners, partly because they have very sensitive steering geometry to make them 'flickable'.
A touring bike is the other end of the spectrum. Handling and even top speed is sacrificed to comfort. The bike is larger because it will be expected to carry luggage, and maybe even ride two-up most of the time. There are other features that make it better for long trips like a bigger gas tank, bigger fairing to keep off the wind and weather, and wider, less aggressive tires which last longer.
Sport-touring was practically invented by BMW in the 70s and 80s, the idea of spending your vacation in the Alps, riding from one hotel to another. And that's a real compromise between sporting qualities and touring qualities. But the category 'sport touring' just sounds so good that most 'standard' bikes today are advertised as sport-tourers. Some of them are more at the sporty end of that definition, some are more at the touring end.
Some touring bikes (like the Honda Gold Wing) are no fun in town. At low speeds they are heavy and ponderous, like driving a truck. Real sport bikes are not exactly ideal for town driving either, partly because of the position, you're looking down at the street and can't see traffic.
I just thought this may help some. I am sure some will disagree but for me this pretty much covers it.
Todd