The diagram below represents the inside of the mouth. 

The vowels towards the left are front vowels. The tip of the tongue pushes forward towards the front of the mouth.

The vowels towards the right are back vowels. The tip of the tongue pulls back.

The vertical axis represents the height of the tongue. The lower the axis, the more open the jaw is. The vowels closer to the roof of the mouth are considered close-vowels, and the vowels lower down are considered open vowels.

 

Source: https://commons.wikimedia.org/wiki/File:RP_vowel_chart_(monophthongs).gif

 

Phoneme

Position

/ae/

Near-open front unrounded vowel

/ɪ/

Near-close near-front unrounded vowel

/ʊ/

Near-close near-back rounded vowel

/ɛ/ (/e/)

Open-mid front unrounded vowel

/ɒ/

Open back rounded vowel

/ʌ/

Near-open central vowel

/ə/

Mid central vowel

 

Phoneme

Position

/iː/

Close front unrounded vowel

/ɜː/

Mid central vowel

/ɔː/

Mid-back rounded vowel

/uː/

Close back rounded vowel

/ɑː/

Open back unrounded vowel