The acoustic guitar is an instrument which produces sound via the vibration of the strings through the hollow body, sound board and sound hole. String vibration can be achieved by using a flatpick, usually made of plastic, or by simply strumming the strings with the thumb. One can pick individual notes or strike a combination of strings at once to produce a chord.
A more intricate method is fingerpicking, a technique which uses the thumb and tips of the fingers to strike individual strings. Fingertips, fingernails (which are usually grown longer for this specific purpose), fingerpicks which are attached to the thumb and fingers can all be used for fingerpick guitar playing.