Italic
A set of letters is normally called an alphabet.
It's called fan mail fan letters
A snow vehicle with the letters kid in it is called a Skidoo
Scrabble letters are called tiles. The part that holds each player's letters is called a rack.
Letters can be sent to an address.
If you're referring to St. Paul's letters in the New Testament, they are called epistles.
A group of postal letters is called a mail or a batch of mail.
If the letters represent unknowns they are usually called variable.
The letters in music are called the musical alphabet. The letters represent notes, and there are 12 of them. Natural notes are given the letters A through G. The other notes are in between natural notes and they are called sharps and flats.
These letters are referred to as the Epistles.
A word. Many groups of letters is called a sentence(:
A narrow valley with 4 letters is called a "rift."