The letters A, E, I, O and U are called vowels. (The other letters in the alphabet are called consonants.)
More about Vowels
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". So what? Well, using this definition, the letter Y in words like hymn and shy is also a vowel. However, in words like beyond and yes, Y is a consonant because the breath is partly obstructed. So, is Y a vowel? Sometimes it is, and sometimes it isn't, which is why it is often called a semi-vowel. The argument for classifying Y as a consonant (which most do) is based on this: When Y is a vowel, it's really just an I. When it's a consonant, it's being itself.
- The word Iouea (a genus of sea sponges) contains all five vowels and no other letters.
(Being the name of a genus (i.e., a proper noun), it is written with a capital letter. Also of note, it is the shortest word with four syllables.)
- The words abstemious and facetious contain all five vowels in order.
Why Should I Care about Vowels?
Use an (not a) before a vowel sound. The important word here is sound. (The ruling is not use an before a vowel.)
While we're on this subject, it's worth reminding ourselves that the words an and a are called the indefinite articles.
- an apple. [correct] a apple. [wrong]
(An is correct because apple starts with a vowel sound (and a vowel for that matter).)
- an RTA. [correct] a RTA [wrong]
(An is correct because RTA starts with a vowel sound (ar), even though the first letter is not a vowel.)
- An unidentified man with a unicorn tattoo rented a house an hour ago.
(Even though they start with the same three letters, unidentified and unicorn attract different indefinite articles. Similarly, hour attracts an while house attracts a. It's all about the sound of the first letter.)
- Use an if the next word starts with a vowel sound.