Difference Between STUFF Vs REPLACE

STUFF - Delete a specified length of characters and insert
another set of characters at a specified starting point.

ex: SELECT STUFF('abcdefghi', 3, 3, 'ABC')
Go
Answer is: abABCfghi

REPLACE - Replace all occurrences of the second given string
expression in the first string expression with a third
expression.
For Example: SELECT REPLACE('Abhay', 'a', 'v')
Answer is: vbhavy