Stuff function: - This function is used to replace string from the given start position, passed as 2nd argument with string passed as last argument. In Stuff function, 3rd argument defines the number of characters which are going to be replaced.
 
Syntax:-

STUFF ( character_expression , start , length , replaceWith_expression )

For example:-

  Select Stuff ('Software', 3, 3, 'abc')

This query will return the string "Soabcare". In this example, Stuff function replaces the string "Software" onwards the 3rd position('ftw') with 'abc'.

Replace Function :- Replace function is used to replace all occurrence of a specified with the string passed as last argument.

Syntax :-

REPLACE ( string_expression , string_pattern , string_replacement )

For example:-

Select Replace ('Abcabcabc', 'bc', 'xy')

This query will return the string Axyaxyaxy. In this example, Replace function replaces the occurrence of each 'bc' string with 'xy'.