An ELT Glossary : Vowel sounds

Definition : Vowels are phonemes made without obstruction of the airflow as it passes out from the lungs through the mouth. This contrasts with consonants, which involve some obstruction, complete or partial, in the vocal tract.

Examples : The vowels used in English may be :

monopthongs : eg /i:/  as in deep /di:p/
dipthongs :  eg /ɪə/ as in near /nɪə/ 
tripthongs :  eg /aʊə/ as in our

Vowels are described by the shape of the lips (rounded/spread) position of the tongue : how far it is raised (open/mid/close) and which part is raised (front/centre/back) :

Rounded / Spread vowels
Say /u:/  and then /i:/ and notice the difference in the shape of the lips. /u:/ is made with the lips in a rounded position, whereas /i:/ is made with the lips spread.

Open/Mid/Close vowels
A close (NB./kləʊs/ not /kləʊz/) vowel is made by raising part of the tongue close to the hard or soft palate. Both /i/ and /u/ are close vowels. 
This contrasts with open vowels such as /æ/ and /ʌ/ where the tongue is low, whereas for mid-vowels such as  /ə/ tongue position is half-way between open and close. Try saying /ɪ/ /ə/ /æ/ and feel the jaw gradually open and the tongue move gradually downwards.

Front/Central/Back vowels
A front vowel is made by raising the front of the tongue, eg /i/
A central vowel is made raising the centre of the tongue - eg/ə/
A back vowel, made by raising the back of the tongue - eg /u/