Adobe illustrator

How to create clipping mask & opacity mask in Adobe illustrator

How to apply clipping mask in Adobe ( ai ) illustrator. According to Adobe – CLIPPING MASK is a group of layers to which a mask is applied”. Suppose you have a shape in the top layer and you have placed an image below it. Then after applying clipping mask you shall be able to see the image only through the boundaries of shape. Other portion of the image should remain hidden. The bottom – most layer defines the visible boundaries of the entire group of layers of shape, images. But description does not clarify a clipping mask. You have to see the result.