A palette is a set of discrete colors. In GIMP, palettes are used mainly for two purposes:

Actually neither of these functions fall very much into the mainstream of GIMP usage: it is possible to do rather sophisticated things in GIMP without ever dealing with palettes. Still, they are something that an advanced user should understand, and even a less advanced user may need to think about them in some situations, as for example when working with GIF files.

When you install GIMP, it comes supplied with several dozen predefined palettes, and you can also create new ones. Some of the predefined palettes are commonly useful, such as the “Web” palette, which contains the set of colors considered “web safe”; many of the palettes seem to have been chosen more or less whimsically. You can access all of the available palettes using the Palettes dialog. This is also the starting point if you want to create a new palette.

Double-clicking on a palette in the Palettes dialog brings up the Palette Editor, showing the colors from the palette you clicked on. You can use this to paint with the palette: clicking on a color sets GIMP's foreground to that color, as shown in the Color Area of the Toolbox. Holding down the Ctrl key while clicking, on the other hand, sets GIMP's background color to the color you click on.

You can also, as the name implies, use the Palette Editor to change the colors in a palette, so long as it is a palette that you have created yourself. You cannot edit the palettes that are supplied with GIMP; however you can duplicate them and then edit the copies.

When you create palettes using the Palette Editor, they are automatically saved as soon as you exit GIMP, in the palettes folder of your personal GIMP directory. Any palette files in this directory, or in the system palettes directory created when GIMP is installed, are automatically loaded and shown in the Palettes dialog the next time you start GIMP. You can also add other folders to the palette search path using the Palette Folders page of the Preferences dialog.

GIMP palettes are stored using a special file format, in files with the extension .gpl. It is a very simple format, and they are ASCII files, so if you happen to obtain palettes from another source, and would like to use them in GIMP, it probably won't be very hard to convert them: just take a look at any .gpl and you will see what to do.