Compare the current file (the file at point) with another file (the file at the mark) using the diff program (dired-diff). The file at the mark is the first argument of diff, and the file at point is the second argument.

Compare the current file with its latest backup file (dired-backup-diff). If the current file is itself a backup, compare it with the file it is a backup of; this way, you can compare a file with any backup version of your choice.

The backup file is the first file given to diff.