Clipping Path is a vector Path with hard and smooth edges. In 2D image design, Clipping Path is used to remove background and create transparency around the image, so that it can be used in catalog or websites without the background.
Clipping Path (or Photoshop Knockout) is the best way in Photoshop Design to remove the background. Again, this is a designer's nightmare, as it takes huge amount of time and concentration. It is somewhat a boring job. Probably, that's why most freelance Photoshop designers don't know what the Pentool in Adobe Photoshop is used for. But, this pentool is one of the best tool for exact masking for images in Adobe Photoshop.
Clipping Path is not a feature of Photoshop alone. It is also used in other Image Manipulation softwares like: CorelDraw, Illustrators, Freehand, GIMP, etc.
Please refer to this tutorial: http://roberttgasperson.com/articleblog/2007/07/28/photoshop-tips-how-to-create-a-clipping-path-on-an-image/ that will help you undertand how Clipping Path is done and how to use the Photoshop Pentool effectively.