Earphones go by many different names including headphones, headsets, earbuds, or sometimes earpieces. http://www.synonyms.net/synonym/headphone http://www.wisegeek.com/what-are-headphones.htm By any name, earphones are small speakers placed in, over, or around a person’s ears to amplify or contain a sound, making it easier for the wearer to hear. Earphones are commonly used to transmit sound from a device like a computer, portable media player, or mobile phone directly in to the ear of the wearer to maintain privacy, so the wearer can hear clearly, and the sound is not heard (or is significantly muffled) to others nearby. http://www.wisegeek.com/what-are-headphones.htm

Similar devices can be used to cancel out external sound to the wearer; these are called noise-canceling headphones, while other devices are used to amplify external sound (rather than that from a source) to make it easier for the wearer to hear; these are often similar to hearing aids and are used in cases of hearing loss. http://reviews.cnet.com/best-noise-cancelling-headphones/

In the past, earphones were larger over-the-ear devices. The size was necessary to contain the internal workings of the earphones. Today, with advances in technology, earphones can be quite small. A standard size of earphones, often referred to specifically as earbuds, are ½” to ¾” in diameter to fit comfortably over the ear canal, secured by the conch, which removes the need for a band over the head. Larger earphones are still made for those who prefer the better sound quality, the noise-cancelling effects, the appearance, or those who can’t wear earbuds due to the shape of their ears. These are often referred to specifically as headphones, and use a plastic, metal, or similar band to connect the two speakers and hold the entire device on the head. http://www.wisegeek.com/what-are-headphones.htm

Either earbuds or headphones can also have a built-in or attached microphone for two way communication. These are often used for gaming or for use with Bluetooth and a mobile phone. http://www.wisegeek.com/what-are-headphones.htm