A vowel is a letter of the alphabet  which includes (a, e, i, o, u, and sometimes y) represents a speech sound. Vowels are the main sounds of syllables. Letters that are not vowels they are called  consonants.
Vowels are a major category of phonemes in English speech.  In spoken English we have approximately 20 distinct vowel sounds, though there are dialectal variations.
From the Latin, “voice”
Examples and Observations
“Written English has five proper vowel letters, A, E, I, O, U (Y may substitute for I). Yet spoken English has some 20 shades of vowel sounds. Accordingly, our vowel letters are kept busy, each one symbolizing multiple sounds on any written page. Our letters get some help from rules of spelling, which, for example, can specify the long A of ‘rate’ versus the short A of ‘rat.'”
“In all vowels, the mouth passage is unobstructed. If it is obstructed at any time during the production of a speech-sound, the resulting sound will be a consonant.”
“In written English, . . . the 26 letters of the alphabet comprise 5 vowels and 21 consonants. In spoken English, there are 20 vowels and 24 consonants.
Vowels in Dialects
“How many different vowels does English have? Well, it depends on your dialect. Standard American English makes fewer vowel distinctions than Standard Southern British English.
The Northern Cities Vowel Shift
“The change which has become known as the Northern Cities Vowel Shift, which is typical of the accents of younger people (except for African Americans . . .), particularly in cities such as Chicago, Detroit, Cleveland and Buffalo, . . . is a kind of circular movement of vowels which represents one of the most dramatic changes ever to have taken place in the history of the pronunciation of English.
In this change, the vowel of nought and all similar words (caught, taught, law, falletc.) is moving downwards to the position of gnat. The /æ/ vowel of gnat has dramatically altered its pronunciation to the kind of /ǀə/ quality described also for New York City . . .. The vowel of net, meanwhile, has moved in the direction of nut, which in turn has got out of the way by moving in the direction of nought, thus completing the circle.”
Never Trust a Vowel
Vowels were something else. He didn’t like them and they didn’t like him. There were only five of them, but they seemed to be everywhere. Why, you could go through twenty words without bumping into some of the shyer consonants, but it seemed as if you couldn’t tiptoe past a syllable without waking up a vowel.
Consonants, you knew pretty much where you stood, but you could never trust a vowel.”