When looking at flamenco guitars you'll find that they are deeper and that the woods are a little bit thicker in comparison to classical guitars.
You'll also find that the necks feature a flat or even negative relief so as to create a very fast action even though it will result in some buzzing which is a huge no-no in classical guitars.
This buzzing however has become so common that it's now just accepted as a part of the flamenco sound.
A further big difference is the distance between the strings, that is to say, how far apart the stings are spaced between the two types of guitars.
Flamenco guitars are spaced wider at about 64mm as this allows the hand to move faster and facilitates many other flamenco techniques required to execute a repertoire of songs.
Such techniques would include Alzapua, Rasgueos and Picado.
While the bridge of a flamenco guitar is flatter it will also feature a thin, plastic tap-plate just below the stings on the body of the guitar.
This is protect the guitar's surface from golpe or hits which again make up an enormous part of playing flamenco music.
Sound wise a well constructed flamenco guitar will have a very quick response and the sound will also decay quickly.
This is in stark contrast to classical guitars whose sound is designed to sustain.
When playing flamenco the notes are often produced in flurry's and so a quick decay helps to maintain clarity.
Should the guitar sustain the sound could become very muddy and dark.
All in all a flamenco sound should be percussive and bright with few overtones.
It should punch through the air with the volume required to be heard over dancers, castanets and claps.