GIMP (which stands for GNU Image Manipulation Program) or "The GIMP" as it is sometimes called, is a powerful image processing tool. GIMP is quite similar to commercial software such as Adobe Photoshop, except as open source software, it's completely free for anyone to use.
GIMP allows you to modify and adapt your images in many ways. You can use its basic features for causal use such as re-sizing and cropping images, adjusting the brightness and contrast, and adding text to graphics. GIMP also has many advanced features allowing you to apply various effects and filters, work with layers for things such an animated GIFs, or optimize images for print or the web.
Anyone from casual users to professional designers and image manipulators will find GIMP useful. You might find it has more features than needed if you're just cropping your holiday snaps, but you won't find it lacking if you're designing print or web material.
GIMP can be installed on Windows, MAC, and Linux. If you're using an Ubuntu or a GNU/Linux Operating System then you are in luck, as GIMP is frequently included by default!