Vowels & Consonants made up the speech sounds (or phonemes) of English.

Vowels, unlike consonants, are produced without any significant obstruction or constriction of the airflow from the lungs to the lips and beyond.
(Thornbury Scott An A-Z of ELT pg. 241)

Vowels are produced when the airstream is voiced through the vibration of the vocal cords in the Larynx, and then shaped, using the tongue and the lips to modify the overall shape of the mouth.
(Gerald Kelly – How to teach pronunciation pg. 5)

What shapes the vowels and distinguishes them from one another are:
          – the position of the tongue
          – the shape of lips
(Thornbury Scott An A-Z of ELT pg. 241)

 The position of the tongue is a useful reference point for describing the differences between vowel sounds.
(Gerald Kelly – How to teach pronunciation pg. 5)

With regards to the TONGUE, the significant features are:
            – the PART of it that at it is raised to
            – the HIGHT that it is raised to.

If the front is raised, the vowels that are produced are called Front Vowels.

If the back is raised, the vowels are called Back Vowels.

Central Vowels fall somewhere in between.
(Thornbury An A-Z of ELT pg. 241)

The below diagram is a representation of the ‘vowel space’ in the centre of the mouth where vowel sounds are articulated.