Transparency and opacity in image combinations can make image parts seem to gradually change from one object to another or just look like they are adjacent to one another. You can also make it appear as though smooth or hard-edged shapes are cut out of an image combination — like you might need to do at the bottom of a Web site banner graphic that contains multiple photos, for example. Another common use for transparency is simply knocking out the background behind people or objects, or silhouetting.
You can make smooth, hard-edged transitions or very gradual transitions between transparent and opaque areas. The transitions can be painted on in a random or flowing artistic way or they can be made in a very precise way that runs along the edge of an object.
You can use numerous methods and tools in Photoshop to make parts of images transparent or partially transparent. There are a few common good practices to use when you are doing this, though, and they give you a lot of flexibility when working with transparency and opacity.
First, it's a very good idea, and in many cases necessary, to put the objects in an image on their own separate layers. When an object is on its own layer, you can activate the layer in the Layers palette, and you will be working on the object that is on that layer. If all of your objects were on one layer, you would have to select an object and work within the selection, which would in most cases turn out to be a whole lot ...