XCF is the native image format used by GIMP - The GNU Image Manipulation Program. XCF files support layers, channels, transparency, and path data. They use .RLE compression by default, but can also be compressed using .GZIP or .BZIP2 compression.
GIMP is a free image editing program design primarily for manipulating photos. The program was originally developed for Unix-based systems, but Mac and Windows versions are now also available. "GIMP" initially stood for "General Image Manipulation Program," but it was later changed to the "GNU Image Manipulation Program" after it became part of the GNU project.