A clipping mask in Photoshop is a group of layers to which a mask is applied. The bottommost layer, or base layer, defines the visible boundaries of the entire group. For example, if you have a shape in the base layer, a photograph in the layer above it, and text in the topmost layer, if the photograph and text appear only through the shape outline in the base layer, they also take on the opacity of the base layer. Clipping masks are useful for controlling the visibility of a layer and are similar to layer masks, but they use the content and transparency of one layer to control the visibility of another.
To create a clipping mask in Photoshop, you need at least two layers. You can create a clipping mask by selecting the existing layer in the Layers panel, then choosing New Layer from the Layer menu or the Layers panel More menu. In the New Layer dialog box, select Use Previous Layer To Create Clipping Mask, and click OK. Alternatively, you can place the layer you want to clip on top of the layer you want to "clip" it to in the Layers panel.
Its important to note that not all the layers above the base layer of the clipping mask will be affected - only the ones belonging to the clipping mask. To identify these layers, you will see that they are indented and have a down arrow/clipping mask icon next to them.
