Basically, a vowel is any "open" sound where there is no obstruction or "blocking" caused by the teeth, tongue, lips, palate or other articulators. In the English alphabet, there are 5 vowels: A, E, I, O, U. But there are many more vowel sounds in the English language. For example, the /e/ vowel sound is usually represented by the letter "E". But when you put two "E" letters together, like in "speed" (/spi:d/), you get a long vowel sound: /i:/.
A single vowel sound is any vowel that is not a diphthong (see below). A single vowel can be short or long.
This is the list of the short vowels in standard (RP) English:
The schwa is a special type of short vowel. It is a very "weak" sound that is never stressed. This means you often find the schwa in words with more than one syllable. Here are some examples:
In the British English phonemic chart, long vowels are easy to recognise, because they have a colon (":") symbol after them. Some long vowels are basically longer versions of short vowels (like /ɪ/ and /i:/).
Long vowels in English:
A diphthong is a two vowel sounds, one after the other. There is movement or "glide" between the two parts of the sound. For example, to say the /eɪ/ dipthong, like in the word "cake" (/keɪk/) first say /e/, then say /ɪ/ without stopping. Your mouth will move from the /e/ shape to the /ɪ/ shape. This is the "glide".
Diphthongs of English: