Homophones normally come in pairs. A homophone is a word that sounds the same as another word, but has a different meaning, and usually a different spelling. 'Bare' and 'bear' are one pair of homophones; 'there', 'they're' and 'their' form a triplet (group of three). Homophones are the basis of puns.
For the opposite phenomenon, words that look alike but sound different, see homographs.
For a list of examples of homophones, see category:homophones.