What is a vowel sound?
It is a sound in which there is a continuous vibration of the vocal folds and the airstream is allowed to escape from the mouth in an unobstructed manner, without any interruption.
In Spanish, we have five letters corresponding to vowel sounds, namely, /a/, /e/, /i/, /o/, /u/. In English, we obviously have the same letters, but there are 12 different vowel sounds also called monothongs, 8 diphthongs (combination of two different vowel sounds) and 5 triphthongs (combination of 3 vowel sounds).
Vowel classification
We can classify the vowel sounds considering four different criteria:

TONGUE HEIGHT: It refers to how high or low our tongue is. Thus we can say that /i/ is a high vowel sound, /e/ is intermediate, and /a/ is low. You may notice the movement of the tongue if you pronounce these three sounds.

TONGUE POSITION: When we make a vowel sound, our tongue is in a specific position inside our mouth. For example, if you make the vowel sound /i/ and then /u/, you will notice that your tongue moves back, and if you make the sound /i/ and then /a/, your tongue moves down. The following chart shows the position of the tongue when we pronounce the English vowel sounds. You have to imagine that this chart is inside the mouth. For each sound, there is one phonetic symbol that belongs to the International Phonetic Alphabet.

LIP ROUNDING: It refers to the position of our lips, when we make the vowel sounds. When we make the sound /i/, the corners of our mouth are further apart, so we can say that this is a spread vowel sound. When we pronounce /u/, our lips are rounded, and for the sound /a/ our lips are neither spread nor rounded, so we say that it is an unrounded vowel or neutral.

As you can notice in the chart, there are two types of /i/. One of them has a colon (:), which means that it is a long sound. So there is a difference you may perceive when you hear the words heat /hi:t/and hit /hIt/. The first one is longer. The same happens with the other sounds that you can see in the chart.
Summarizing, we can give different labels to each vowel sound. For example, the sound /i:/ is front, high, long and spread whereas the sound /i/ is front, high, short and neutral. Click here to see a powerpoint on