Create & Drop Shadows
Generally shadows are classified in Hard and soft, what differentiates hard shadow from Soft shadow is the contrast of the shadow and sharpness of its edge. Hard shadows have more contrast and sharp edges and soft shadows have less contrast and less edge definition.
Shadows are a part of everyday life and, it sometimes seems, a part of most images we see in print or on the Web. Shadows are most often decorative in nature, working to enhance an image. Shadows, on the other hand, are usually meant to provide realism or depth. Shadows are most common with text and buttons, giving the illusion that the letters or the shapes are floating above or in front of a background or image. Shadows are typically created for objects that stand on their backgrounds. An example would be a person standing on a floor. Shadows can (when applied properly) add depth to an image.
Shadow can be defined by the absence of light caused by the interposition of an object between the source of light and the surface that receives the light. Shadow must not be perceived as a projected darkness but as the absence of light. Objects do not cast shadows but their opacity prevents light from being diffused. We often think of shadows as a dark counterpart of an object which is only a mind created illusion. In reality shadows are not part of a dualistic system of light/darkness but only the absence of light.