A letter of some alphabet (either upper case or lower case), a digit, a punctuation or other symbol or a control character. In a computer, a character is represented as an integer. What character is represented by what integer is determined by the current character set. For example, in the ASCII character set, "A" is 65. These integers are then stored as a sequence of bytes according to a character encoding.
The character set and encoding is usually implicit in the environment in which the character is being interpreted but it may be specified explicitly, e.g. to convert input to some standard internal representation.
A sequence of characters is a (character) string.
Compare with glyph.