In a car, the exhaust system is simply a utilitarian device meant to process the exhaust and reduce its environmental effect and noise. Motorcycle exhaust systems are different. Motorcycles often have chromed, decorative exhaust tubes that port the gasses away from the engine. Rather than having a single tube to port the spent gas from the whole engine, a motorcycle may have two or even four tubes flowing from the different cylinders. This increases gas flow, making the motorcycle perform more efficiently. More importantly, it looks great.
When the engine expels hot gasses, they are routed into tubes. They flow down in one, two, or four tubes into the catalytic converter. The catalytic converter contains a catalyst, a substance that makes it easier for certain chemical reactions to occur. For example, it turns much of the carbon monoxide in the exhaust into carbon dioxide, a less harmful gas. After that, it flows through to the muffler. The muffler has a resonating chamber specially designed to cancel out the sound waves from the motorcycle engine. The waves reverberate around in the chamber, partially canceling each other out and reducing the sound of the motorcycle.
Some motorcycle drivers like their bikes to make a lot of sound. It makes them more noticeable to cars, and gives their bikes a theatrical roar. A motorcyclist gets this sound by installing a muffler that provides less noise cancellations. For example, a cherry bomb is a straight metal tube with holes in it covered by glass to insulate some of the sound. There is no resonating chamber at all, so the motorcycle makes much more noise.