The clipping group used in the layers is often confused with the well-known term clipping path, but they are two completely different things. The only common point is their effect: a group of off putting your picture in a text (or whatever space) and in the next lower layer, we can say it removes everything except the limits of the text.

A clipping path is created with the Pen and you can save this path so that when you insert the image into another application (like QuarkXPress, the Adobe InDesign, the Adobe Illustrator, etc.), you can cut anything that is outside the path. This technique is often used for white backgrounds that lie behind objects.

As a conclusion, remember that the clipping groups is a layers working technique, the clipping path is a path created with the Pen tool and that it is used to separate images from their background.