Earphones are a compact, cheap and portable way to experience sound. Unlike headphones, which are fairly large due to the brace which connect the two speakers, earphones are around 1cm in diameter and moulded to sit just inside each ear, for stereo sound.
Earphones usually have a 3.5mm stereo plug, making them easy to connect to portable audio devices like radios, CD players, MP3 players, as well as sound cards, portable stereos, etc.
Earphones are very practical to use where size is an issue with portable devices because they are so compact. The sound is excellent considering the size are price. Not as good as expensive headphones or hi-fi speakers, but still very good for portable applications.