Vowels in the English language are A, E, I, O, U, and sometimes Y.
The term "cardinal vowels" is related to a system created by Daniel Jones to classify the ways in which the parts of the mouth and throat involved in articulation are shaped to make the different sounds associated with those vowels, separately or in combinations. The voicing of vowels is affected by the height of the space inside the mouth (between the roof of the mouth and the jaw), the backness (where the tongue is placed during the pronunciation of the vowel), and the roundedness (the shaping of the lips).
Because vowels in English have many different sounds, depending upon their placement and combination with other letters, it is not possible to give one all-encompassing classification of any given vowel. The pronunciation depends upon too many variables.