SQL Server

The REPLACE function replaces all occurrences of a substring, within a given string, by a different string.

In contrast, the STUFF function deletes a specified number of characters starting at a specified position within a given string and then inserts at the deletion point a different string.

What this boils down is that REPLACE replaces all occurrences but STUFF only replaces a single occurrence of one string by another string within a given string.