Keyboard shortcut
M
toggle between

and

+
shift ⇧ =

+
opt ⌥ =
Elliptical marquee makes an elliptical selection.
To draw a circle, hold down the Shift ⇧ key after you begin dragging.
To drag a marquee from its center, hold down Option ⌥ after you begin dragging.
Tool Options
You can use the following marquee modes:
New - to draw new selection.
Add To - add to border of existing selection.
Subtract From - substract from border of existing selection.