If you say 'turbo' everybody gets that grin on his face, but do they know how it really works? A turbocharger is a simple mechanical device that uses a compressor to force more air into the engine. The more air in, the more power you get out. It's a mechanical version of a forced airbox, like all sportsbikes now come with.
A turbo has two turbines in the motorcycle's exhaust system. As the exhaust gases escape the engine at speed, they turn these turbines, which in turn drive a compressor. This compresses the air and feeds it into the engine, making more power.
The neat thing is that the faster the engine revs, the faster the exhaust gases will spin the turbines, creating more compressed air and more power. It's almost self-perpetuating.
So why aren't turbo-charger's more common on motorcycles? While most cars, and all diesels, are turning towards turbos, the popularity of motorcycle turbos never recovered after the high profile failures of the early '80s. This is a shame, because turbos are a very efficient way of increasing both power and torque. The problem with fitting a turbo-charger to motorcycles is their size.
A turbo needs a lot of cooling to work efficiently, as well as extra pipe work and the actual turbo unit, which all adds to the weight. Motorcycles are designed to be compact, so fitting a turbo-charger requires careful placement.
Then there's the myth of lag. While turbo lag existed in the '80s, modern systems have all but banished it, not that your know-it-all friend down the street will acknowledge this. There's a lot of bullshit surrounding turbos, but the fact of the matter is that you can turbo a Suzuki Hayabusa and get a 250 horsepower motorcycle that is totally ridable and reliable and probably more fuel efficient than a normally aspirated one.