DIFF is an extension specific to DIFF that allows you to compare two text files on a line-by-line basis. Files with the DIFF extension are the output data generated by comparing two files. It is a text file in which the "-" sign is used to mark lines appearing in only the first file, and the "+" sign is used only in the second file. Common lines are not marked.

DIFF files can alternatively be given the PATCH extension. These are technically plain text files.