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EyeSpeak Student Manual:

Vowels:

Vowels are speech sounds that occur in the middle of syllables. Unlike consonants, they do not involve the obstruction of the oral tract. Vowels are identified by looking at their height, backness, roundedness and length.

Height and Backness
High vowels are those where the tongue is high in the mouth, and the mouth therefore somewhat closed. The highest vowels in English are those in "key" and "do".
Low vowels are produced with the tongue low in the mouth, and the mouth very open. The lowest vowel in English is that in "hard".
Front vowels are those where the tongue is forward in the palatal region of the mouth. Front vowels in English include those in "man" and "head".
Back vowels are produced with the tongue back in the velar region of the mouth. Back vowels in English include those in "pot" and "caught".

Roundedness
In rounded vowels the lips are tensed and rounded, as though one is about to whistle.
In unrounded vowels the lips are spread.

Rhotic vowels
In rhotic vowels the tongue is curled as though one is making the consonant sound 'r'.

Length
Vowels in English are divided into long and short categories. Long vowels are about twice as short ones. Two small triangles following a vowel symbol indicate that it is long. In English short vowels can never be the final speech sound in a word.

Monopthongs and Dipthongs
Monopthongs are vowel sounds where the tongue stays in on place for the whole sound. All the vowel sounds already mentioned are monophthongs.
In dipthongs the tongue moves in the middle of the vowel sound, as in "say" or "boy". Dipthongs are represented by two symbols beside one another: The symbol of the start position of the tongue and the symbol of the finish position. In English all diphthongs are long.