Color keys are specified color values of a surface's pixel format that are given special treatment depending whether it is a destination or source color key. Color keys are usually specified as a single color, but a color range may be used.
The destination color key identifies a color or color range that may be overwritten by other image information. This can be used to give a foreground effect because pixel colors outside the destination key range will not be overwritten, potentially giving the appearance of the transferred image moving behind those pixels.
The source color key identifies a color or color range that will not be copied from the source image and thus not drawn at the destination. This can be used to display images that are not rectangular by drawing all the parts of an image that you dont want transferred in a specific color or color range and then defining that color as the source color key when the image is being transferred to it's destination.