In contrast to consonants, where voice, manner and place of articulation serve as descriptive categories, vowels are differentiated by their position of the tongue and the lips. While consonants are produced with the help of many organs, vowels depend only on the position of the tongue and the lips.
The positioning the tongue and the lips allow for a great amount of variation, enabling us to voice many more vowels than the familiar five vowels a, e, i, o, u. English has more than a dozen different vowels.
Vowels can be produced by raising the tongue high, keeping it in the middle, or low in the mouth (Height, or Closeness/openness). The usual scale is high [i, u], mid[e, o], and low [a]. There may be two middle steps in the ladder, usually called closed [ay, oh] and open [eh, aw].
The tongue can be moved to the front, centre or back (Frontness/Backness). Thus, vowels can be classified into front (i, e), central (a, or the indistinct vowel in 'of'), or back (o, u).
You can arrange the vowels in a grid according to these two dimensions. The bottom of the grid is usually drawn shorter because there isn't as much room for the tongue to maneuver as the mouth opens more.
Source of the picture: zompist.com
To get a feel for these distinctions, pronounce the words in the diagram, moving from top to bottom or side to side, and noting where your tongue is and how close it is to the roof of the mouth.
Vowels can vary along other dimensions as well:
- Roundedness: whether the lips are rounded (u, o) or not (i, e). English doesn't have front rounded vowels, but French and German do (Fr. u, oe; Ger. ü, ö). We also don't have (say) an unrounded u, but Russian, Korean, and Japanese do.
- Length: vowels may contrast by length, as in Latin, Greek, Sanskrit, and Old English; Estonian has three degrees of length.
- Nasalization: like consonants, vowels can be nasalized. French, for instance, has four nasalized vowels.
- Tenseness: vowels can be tense or lax-- Tense vowels tend to have the position of the tongue body slightly higher than the corresponding lax vowels. Lax vowels tend to be more centralized than the corresponding tense vowels (i.e., closer to schwa in the front/back dimension). Look at soot and sit in the diagram.