Gimp is the: GNU Image Manipulate Program.
The GNU parts means that like most Linux software it is open source and freely available. Its developed by programmers around the world for the benefit of others. Because the source is open, anyone can contribute and it is constantly being improved.
It is image manipulation with the freedom of Linux and without the price tag of Photoshop.
GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.
Gimp supports 8, 16 and 32 point bit depth editing.
Gimp will open almost all RAW files using plugins (Rawtherapee, Darktable, Ufraw and others), or you can use these programs and export to Gimp.
This is much the same way that Photoshop uses cameraraw or lightroom.
RawTherapee is a powerful, cross-platform raw photo processing system, released as Free Software (GPLv3). It is designed for developing raw files from a broad range of digital cameras and targeted at users ranging from enthusiast newcomers who wish to broaden their understanding of how digital imaging works to professional photographers. RawTherapee provides a powerful suite of tools for you to produce amazing photos and showcase your creativity.