The Eyedropper Tool defines the foreground / background color.
- In the Toolbox, select the Eyedropper Tool.
- Click somewhere on your image to define the foreground color. Look how foreground selector on the Toolbox changes.
- Alt-click (click holding down the [Alt] key)
to set the background color. Look how the background selector on the Toolbox changes.
- In the Options bar, you can change sample size (the Eyedropper's range):
- the "Point Sample" picks up the color from the pixel you click on
- all the other samples read average values from larger areas (movie).
Tip. Just in case you didn't know, the Eyedropper tool is also available in Photoshop Elements that is way cheaper version.
And if you believe, Photoshop Elements lacks some other important feature, be sure to visit this page.