In computing, diff is a file comparison utility that outputs the differences between two files. It is typically used to show the changes between a file and a former version of the same file. Diff displays the changes made per line for text files.-In computing, diff is a file comparison utility that outputs the differences between two files.　It is typically used to show the changes between a file and a former version of the same file.　Diff displays the changes made per line for text files.