At a basic level, the role of a headphone amp is a simple one: to amplify one or more input signals in order to drive one or more pairs of headphones. All headphone amps therefore share the goal of doing this with the minimum of colouration and noise, though as with any other piece of equipment you tend to get what you pay for.
A wide range of different headphone amps are available, largely because headphones are used in so many different situations. Just a few of these include: studio use (in both live areas and control rooms), MIDI sequencing, live recording, live sound reinforcement, broadcast sound and DJ use. Beyond the world of music and audio recording, headphones are also used in such diverse areas as school language labs, museums and conference venues. And, last but not least, many people use them to listen to music at home.