They are trained to recognize an object in a region of an image by its color pattern. They can events when the object first appears, and when it disappears. Technically, they match the color histogram of a region of an image with the histogram obtained during training. Since two histograms will never match exactly, there is a threshold on the similarity of the two histograms. The amount of variation which the target can tolerate in the object's appearance depends on how that threshold is set.
More complex targets can respond more subtly to the user's touch.
These provide continuous parameters describing how the user is touching the target. The quality of touch can map to control functions in various ways. Targets are trained by the user identifying a region in the image, typically a rectangle that can be moved and resized. The user then demonstrates the action they want the target to recognize. Options are available to allow the user to configure the events generated by the target.