In general, a "character" is any mark or symbol that can appear in writing.
A "letter" is a character that is part of an alphabet. Basically, a character that represents a sound in the language and that can be combined with other characters to form words.
So in English, the letters are A-Z, in both capital and small versions. Characters include the letters, and also punctuation marks like a period or comma, and other symbols included in writing, like a dollar sign.
Note that in the computer world, "character" has a somewhat more specific technical meaning: it's a value from the "character set" represented by a code and that can be stored in a character or string variable. The old ASCII character set includes a number of "non-printable characters", control codes that were sent between devices. The idea of "non-printable characters" doesn't make much sense in conventional writing and printing.