A monophthong is a single vowel sound - the monophthongs used in English, with examples of the words containing them, are :
A diphthong 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 diphthongs used are :
A triphthong is a consecutive sequence of three vowel sounds which glide into each other. In British English, the following triphthongs are found :
In many varieties of English triphthongs 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