A normal diff between 2 text files enables you to quickly determine 2 things - Whether the two input files are identical, and if not, exactly where the differences lie.

A Perceptual Diff (pdiff) allows you to do the same with 2 image files and quickly identify whether the two images are identical, and highlight areas where the 2 images mismatch. pdiffy does exactly this.

via kennychua/pdiffy.