Vowel is a single sound of a particular type - it is an open sound which is made with free passage of breath. For example the sound of "ah...' a doctor asks you to make when examining your throat. Articulation of a vowel is not accompanied by any closure or narrowing in the speech tract which would prevent the escape of the air stream through the mouth.
In English language sounds of vowels are represented by the letters a, e, i, o, u, and sometimes w and y. Sound of vowels is distinguished from that of consonants, which are made by organs of speech more or less closed.
Once we understand what is a vowel, a diphthong can simply be defined is sound produced by pronouncing two vowels as a single syllable. A diphthong has a starting point as one vowel and the tongue glides in the direction of the second vowel. The change in sound required for diphthong can also be achieved by change in shape of lips. An example of diphthong is the word kite, where the syllable after the letter k starts with sound of a as in far, and then changes in sound of e as in be. The combination of letters ei in the word height produce similar sound. The letters ou in the word sound represent diphthong with a different sound.
In English language presence of two letters representing vowels generally spell a diphthong. But there are exception to this rule. For example the "oo" in spelling of book and "ai" in spelling of paid represents singles sounds that do not require change in position of tongue or lips in between pronunciation of these sounds. Vowels like these are sometime called long vowels. For example the word mat is pronounced with a short vowel, and the word mate uses the long version of the same vowel. In some words a single letter representing vowels is used to spell a diphthong - for example, the letter i in the word light.
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