What is the STUFF function and how does it differ from the REPLACE function?

STUFF function to overwrite existing characters. Using this syntax, STUFF(string_expression, start,
length, replacement_characters), string_expression is the string that will have characters substituted,
start is the starting position, length is the number of characters in the string that are substituted, and
replacement_characters are the new characters interjected into the string.
REPLACE function to replace existing characters of all occurance. Using this syntax
REPLACE(string_expression, search_string, replacement_string), where every incidence of
search_string found in the string_expression will be replaced with replacement_string.