Keyboard shortcut
M
toggle between

and

+
shift ⇧ =

+
opt ⌥ =
Rectangular marquee lets you select rectangles.
To draw a square hold down Shift ⇧
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.