The letters A, E, I, O, U are the called vowels. The other letters in the alphabet are called consonants.
Use an before a vowel sound, and use a before a consonant sound.
She was injured in a RTA.
She was injured in an RTA.
(The letter R is a consonant, but it starts with a vowel sound.)
Using an or a with consonants
Glossary of grammatical terms