An ELT Glossary : Monopthong / Diphthong / Tripthong

A monopthong  is a single vowel sound - the monopthongs used in English, with examples of the words containing them, are :

/æ/ hat
/ə/  met
/i:/  read
/ɪ/   ship
/ʊ/  book
/uː/ too
/e/  bed
/ə/  reader
/ɜ/ː bird
/ɔː/ bore
/ʌ/  luck
/ɑː/ card
/ɒ/  cod

A dipthong is a vowel sound which, within a specific language, is usually considered to be a "single" sound, but actually consists of two vowel sounds occurring consecutively in the same syllable, which "glide" into each other. In English, the dipthongs used are :  

/ɪə/ near
/eɪ/ play
/ʊə/ pure
/ɔɪ/  boy
/əʊ/ go
/eə/ bear
/aɪ/  high
/aʊ/ how

A tripthong is a consecutive sequence of three vowel sounds which glide into each other. In British English, the following tripthongs are found :
/eɪə/  player
/aɪə/  higher
/ɔɪə/  oil
/əʊə/ lower
/aʊə/  our

In many varieties of English tripthongs tend to be avoided by the use of an "intrusive" consonant - eg our will be pronounced /aʊwə/ rather than /aʊə/ and player as /pleɪjə/ rather than /pleɪə/. However, in other varieties of English, eg Australian English, they are common. 

See also : Vowel sounds