Headphone amplifier

As an own stand alone unit, a stereo headphone amplifier has the purpose to perform a cleaner and better sound quality than a common stereo amplifier, mp3 player or other consumer audio products with a headphones output. Most high end/hifi units use class A amplifier circuits and some use vacuum tubes.
One of the things to consider when buying a headphone amplifier is power. Standard headphones generally use between 10 and 20 milliwatts of power. A headphone amplifier that provides at least 500 milliwatt per channel will usually be enough to power these headphones. However, some of the more advanced headphones use more power, so you will need to choose the amplifier accordingly.

Link:
About a known diy headphone amplifier - Wikipedia article.

Multi channel headphone amplifiers (sometimes also called headphone distribution amplifiers, are intended for the recording studio and "live" studio recording, so that each of users have headphones with a matching volume. Four channels is the most common.
Since the monitoring situation can be different for each of the headphone users, these products may also have separate stereo inputs for each cahnnel, to be connected from a mixing boards monitor send outputs, in order to be able to have different sub mixing levels for each user's purpose.