Growing up, I always thought nylon string guitars are for classical music and all nylon string guitars are alike. But later realised there are distinct differences between the guitars and nylon string guitars are predominantly of three types, 1. guitar that is predominantly made for classical music, 2. guitar that is primarily used in flamenco, 3. guitar that serves both the purposes to some degree.
Classical music is such that requires a melody to be played over a harmony, sometimes a counterpoint, meaning two different melodic lines, but either way, the music is more tone dependent and hence requires the notes to resonate for longer. The weight of the guitar, type of wood, etc have a huge impact on resonance, and usually the darker the wood on the guitar body, the heavier and more resonance offered. Rosewood and Mahogany are the two most common woods of which classical guitars are made.
The Flamenco Negra guitar falls in between the classical and pure flamenco guitars. The side and back of a Negra guitar is made of denser wood such as rosewood and a lighter spruce top to give a mixture of decent sustain but a thinner percussive sound.
These guitars are purely made for flamenco playing, with low sustain but a bright punchy tone, perfect flamenco playing. These guitars are very light. Usually the back and side of this guitar is made of Cypress and a Spruce top to give that kind of tone desired in flamenco music. They look much brighter than the classical guitars, identifying the color is one way to remember the differences between nylon string guitars.