A Char value (or more commonly, "char", or "character") represents a single character value. A char value is not capable of holding any more than one character at a time.
A string is technically a collection of chars. For example, the string "Hello, World!" consists of a total of 13 characters.
Chars can be used in logic statements (using the equals-comparison (==) and the not-equals-comparison(~=)).
For a more in-depth and concise explanation of types, please consult the Lua 5.1 Official Manual.