Structurally, the most prevalent differences are in the neck joints, the bridge designs, the bracing systems, and the nut widths. The neck of both the classical and flamenco guitar is unified and built into the body at the 12th fret, whereas it joins the steel-string guitar's body at the 14th fret with a dovetail joint. Bridges of classical and flamenco guitars hold the strings on a tie block, where the strings are fed through small holes and then tied. On steel-string guitars, the strings are usually secured by pegs wedged into vertical holes. The bracing on classical and flamenco guitars is usually of a radial-fan system, or some sort of variation of this system popularized by Antonio de Torres. Since the string tension of steel strings is about three times greater than that of nylon strings, steel-string guitars usually employ a stronger, less-efficient 'X' bracing system. (Less efficient because this system does not allow the top to vibrate as freely as it would if braced with the radial-fan system). Lastly, the nut width on classical and flamenco guitars is much wider than on steel-string guitars.
The differences between classical and flamenco guitars lie in their materials, construction and sound. Classical guitars are generally made with either spruce or cedar tops and (most commonly) rosewood backs and sides. Traditional flamenco guitars are made with spruce tops and cypress for the backs and sides. The body of a classical guitar is generally deeper and the woods are slightly thicker. Flamenco guitars have a flat neck relief, making the action very fast. The strings are also closer to the body on flamenco guitars. Flamenco guitars often bear a "golpeador", which is a sheet of plastic mounted to the face of the guitar to protect its finish. The classical guitar is designed to give the soloist the tools to perform poly-timbral music ("An orchestra in a box"), and the attack on these guitars is soft with a longer, gradual decay. The flamenco guitar is designed to cut through the sound of dancers stomping their feet, so the sound is a bit more percussive - a loud sonic burst followed by a swift decay.
Price-wise, a decent solid-top student model starts at $300 whereas a concert model starts at about $2000. Upon inspection you'll find that superior building materials are used to produce a concert model - deluxe machine heads, real bone nut and saddle, rosewood with an often beautifully figured grain pattern, and soundboard material with a tight and uniform grain pattern from the finest quarter-sawn wood. Looking at the label may also provide some clues. A concert guitar will often, but not always, bear the handwritten signature or initials or stamp of the master builder.
The GSI Museum is an enormous online archive of thousands of great guitars that have passed through our doors.