It's a hack that makes GIMP work like photoshop... From the website:
"If you've never used Photoshop before, you may not appreciate my
GIMPshop hack. What I've done is renamed and reorganized GIMP's tools,
options, windows, and menus to closely resemble Adobe Photoshop's menu
structure and naming conventions. Many of the menu options and even
whole menus were recreated to faithfully reproduce a Photoshop-like
experience. After running my GIMPshop hack, you'll find that Photoshop
and the GIMP are strikingly similar."