Whats the difference?

Mufflers 101.
Resonators: The body of a resonator is constructed in three layers: Two thin layers of metal with a thicker, slightly insulated layer between them. This allows the body of the muffler to absorb some of the pressure pulses. Also, the inlet and outlet pipes going into the main chamber are perforated with holes. This allows thousands of tiny pressure pulses to bounce around in the main chamber, canceling each other out to some extent in addition to being absorbed by the muffler's housing. Its dimensions are calculated so that the sound waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust.
Regular Mufflers: Located inside the muffler is a set of tubes. These tubes are designed to create reflected waves that interfere with each other or cancel each other out. The exhaust gases and the sound waves enter through the center tube. They bounce off the back wall of the muffler and are reflected through a hole into the main body of the muffler. They pass through a set of holes into another chamber, where they turn and go out the last pipe and leave the muffler. But creates backpressure in doing so, good for torque, but not good for power. There is also a chamber called a resonator that is connected to the first chamber by a hole. The resonator contains a specific volume of air and has a specific length that is calculated to produce a wave that cancels out a certain frequency of sound. Yes a regular muffler has a portion of its area designed as a resonator.
Low Back Pressure - Glass Pack Mufflers: There are other types of mufflers that can reduce backpressure. One type, uses only absorption to reduce the sound. On a muffler like this, the exhaust goes straight through a pipe that is perforated with holes. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. A steel housing surrounds the insulation.
I had an old Capri that had a 'hot dog' resonator in the middle and a high flow muffler on the back, low revs was a real rumble, but in highway revs it was real quiet. (resonators work on specific sound waves which are affected by revs) Hope this helps. Beaner7102