They can do MUCH more than just edit photos. At their lowest common denominator they are quite alike. Although Photoshop can work with 16-bits per channel (48-bit) whereas at present The GIMP is limited to 8-bits per channel (24-bit), though 16-bit support is in the pipeline. The GIMP does have a few major advantages over Photoshop in that is is multi-platform (Windows, Mac, Linux), opensource, and free.