A character is one unit of a set of symbols, letters and numbers, collectively called a character set.  Common character sets are ASCII (typically used on PCs), and EBCDIC (typically used on mainframes).  Characters can either be single byte, or Unicode (two bytes).  See discussion of "CHAR vs. NCHAR" under "Data Types".  

Examples of characters:

 

Character	Type
f	single byte ASCII character
5	single byte ASCII character
Q	single byte ASCII character
#	single byte ASCII character
말	multi-byte Unicode character