alphabet
All 26 letters in order, from A to Z.
uppercase letter
A big (capital) letter, like A, B, C.
lowercase letter
A small letter, like a, b, c.
vowel
The letters a, e, i, o, u. Every word has at least one vowel sound. Y is sometimes a vowel too.
consonant
All the other letters, like b, c, d, f, g.
alphabetical order
Putting words in the same order as the alphabet, using their first letter (ABC order).