Elliptical Marquee Tool

• To create a selection In the Toolbox, select the Elliptical Marquee Tool; click on the image and drag (holding mouse's left button pressed)
• While New selection option choosen on the Options bar you delete the existing selection by creating a new selection.
• To add an ellipse to existing selection you have either to choose Add to selection option on the Options bar or to press (and hold) [Shift] key while selecting.
• To subtract an ellipse from existing selection you have either to choose Subtract from selection option on the Options bar or to press [Alt] key while selecting.
• To intersect an ellipse with existing selection you have either to choose Intersect with selection option on the Options bar or to press [Shift+Alt] keys while selecting.
• To create an elliptical selection with predefined proportions choose Fixed Aspect Ratio style on the Options bar and set Width and Height values.
• While creating new selection you can constrain proportions to 1:1 (ie. create round selection) by pressing [Shift] key while selecting.
• To create an elliptical selection with predefined size choose Fixed Size style on the Options bar and set Width and Height values.
• While New selection option is chosen and the point of cursor is within selected area the tool looks like this and can move selection (ie. borders of selected area and not image content).
• While New selection option is chosen you can also move selection using keyboard cursor keys.

Tip. Just in case you didn't know, the Elliptical Marquee 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.