Edge Feather

Remember how we used Marqueue and Move Tool? There's an option which photoshop calls "Feather". This is used to select an area with hardness value decreased around the border. You need to give a Feather value greater than zero to use this feature. See below:

1. Select Elliptical Marquee Tool, (press M and chose elliptical from the list).
2. In the options pane, give the feather value 10 and select the left eye of this cute model.

(better save this pic and work with it to compare the results)

3. When you end up selecting and release the mouse, notice that the elliptical selection you made automatically decreases its radius. You'll understand why this happened when you move this selection. Select Move tool (V), pressing Alt key, drag the selection to her right cheek. Press Ctrl+D. Compare your result with the one below:

(Feather + Move Tool)(Move Tool)

See the difference between just moving the selected image and moving it with Feather option. This can also be done by selecting the area first (with Feather still 0) and then giving the Feather radius value from "Select Menu >> Feather..."

Feather tool is used to select the image with weak edge, when moved to another region this gives more realistic blend of the image. (compare the eye on the cheek in the two pics above.)