The steel-strung acoustic guitar has a range of sounds that are unmistakeable and not easily replicated with any other type of guitar. Steel strings produce a different harmonic spectrum from nylon strings, and the hollow body amplifies this sound in a very different way from the electric guitar.
In fact, 'amplify' is really the wrong word - all the energy in the guitar's sound comes from the vibrating string itself, and ultimately from the player. The more efficiently this is transmitted by the body, the faster the sound will decay, which is why the sound of the acoustic guitar is characterised by relatively sharp attack and fast decay.
The guitar has a very broad frequency range. Like a good piano, the richness of the lower strings is a product of harmonics that span most of the audible frequency spectrum. Their characteristic 'zing' is produced by a complex mixture of upper harmonics against the fundamental frequency, and the balance of these frequencies is one of the things that marks out a really good acoustic guitar. Inferior instruments tend to sound 'boxy' at the bottom end, as the upper harmonics are swamped by bass and mid frequencies. The condition of the strings is also very important - the zing can disappear rapidly as they accumulate deposits of sweat and dirt. For this reason, most serious acoustic players change strings very frequently - even before every gig.
The sound of the acoustic guitar can vary considerably, not just with the type of guitar, but also with musical style, choice of strings and playing technique. Acoustic players tend to be divided between those who use a plectrum, and 'fingerstyle' players. Whether strumming or playing single notes, using a plectrum generally produces more volume, and particularly more treble than fingerpicking. Heavier strings also produce more volume, but they can be harder on the fingers, and make techniques such as string bending difficult or impossible.
These days, many playing situations will involve amplification. For this reason, electro-acoustic guitars, which are essentially acoustic guitars with integrated pickup systems, have become very popular. In contrast to electric guitars, these generally use piezo-electric pickups which capture sound through the body, although occasionally, magnetic pickups are also found. Piezo systems aim to capture the sound of the whole guitar, and some models even incorporate an internal microphone for added depth. Because of this, the acoustic properties of the instrument are important even if it will always be amplified, and you never know when you might record, which will almost always be done with microphones.
The type and quality of wood used is one of the most important factors in shaping the guitar's sound. The top and back are generally made of the same wood, and this is usually solid, though the cheapest instruments use 'laminated' wood - a polite name for plywood. Solid wood transmits sound much more efficiently and evenly though - a tight, straight grain is essential here too. The favourite woods are spruce and, less commonly, cedar. The sides and neck of most guitars are made of darker, denser hardwood - rosewood has long been a favourite for depth and balance. Ebony and rosewood are the traditional bridge and fretboard materials.
Lastly, the shape of the guitar also has a considerable impact on sound. Unlike the classical guitar, which has a more or less standardised shape, acoustic guitars come in various shapes and sizes. As a general rule, a bigger body produces more volume and more bass, but smaller bodies tend to give more sustain, and a more immediate response.