The letters A, E, I, O, and U are the called vowels. The other letters in the alphabet are called consonants.
A vowel is classified as a speech sound produced by a comparatively open configuration of the vocal tract, with vibration of the vocal cords but without audible friction.
A vowel sound (but not necessarily a vowel in the actual spelling) will be present in a syllable.
Click to see the vowels in this sentence:
What are consonants?
What is assonance?
What is consonance?
Using an or a with consonants
Glossary of grammatical terms
IT'S ALL ABOUT THE SOUND OF THE LETTER NOT THE LETTER ITSELF
Use an before a vowel sound, and use a before a consonant sound. For example:
Read more using an and a.