Rectangular Marquee Tool (M)
Use this tool to make selections on your image, in a rectangular shape. This changes the area of your image that is affected by other tools or actions to be within the defined shape. Holding the [Shift] key while dragging your selection, restricts the shape to a perfect square. Holding the [Alt] key while dragging sets the center of the rectangle to where your cursor started.
Move Tool (V)
Use this tool to, well, move things. Usually you use it to move a Layer around after it has been placed. Hold the [Shift] key to limit the movements to vertical/horizontal.
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