A monopthong is a single vowel sound - the monopthongs used in English, with examples of the words containing them, are :
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 :
A tripthong is a consecutive sequence of three vowel sounds which glide into each other. In British English, the following tripthongs are found :
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