The bigger the can (the body) of the muffler, and the longer, the mellower the sound. Big mufflers also tend to sound more bassy than small ones.

JASMA - certified Japanese spec mufflers. In other words, they meet sound restrictions for the market there and can be legally equipped in Japan. Buying a JASMA muffler assures you that the sound will be nice, as long as it's brand new. Some secondhands have a tendency to delaminate or rust, depending on the brand (some brands are notorious for this).

Fireball - a popular muffler style. It's a brand name.

Free-flow - a straight through muffler. This gives the most power, but it's also the loudest. Most aftermarket mufflers are free-flow.

Chambered - mutes exhaust sound by making it travel between chambers inside the muffler. The Exhaust loses energy, both sound energy and kinetic energy, from bouncing off of walls and reversing or changing direction. This sacrifices top-end power and some response to give you a relatively quiet exhaust note. Stock chambered mufflers are very quiet. Some aftermarket chambered mufflers are quiet at low rpm, but are louder at high rpms, though not as loud as free-flow mufflers.