The marquee tool

Marquee tool is used to select any thing on the layer in rectangles, ellipses, 1‑pixel rows and columns. And if you see the options bar when you select marquee tool you will find new selection, add to selection, subtract from selection and intersect with selection options. They are very interesting and useful.

First open an image in photoshop, if the file is in GIF format change the image mode to RGB, then only it allows you to edit the GIF image. The draw a rectangle using rectangle marquee tool and then press Ctrl+J (Layer via copy) or Ctrl+Shift+J (Layer via cut). Do the same thing using remaining shapes, elliptical, 1-pixel row/ column marquee tools. Now you should know the basic usage of the tool.

• Now draw a circle using elliptical marquee tool.
• Holding Shift key while drawing anything allows you to draw proportional shapes,
• holding Alt key allows you to draw shapes from its center,
• holding both Shift and Alt keys allows you to draw a shape from its center where you started drawing and also the proportional shapes,
• holding space bar allows you to move the shape completely while drawing itself,
• practice this several times, it really helps.

Okay, now you drew a circle using elipthical marquee tool at the center of your canvas.
Now draw one more circle overlapping the existing selection by selecting the four options( new, add to, subtract from and intersect) one by one. You will notice the difference. Make sure the feather is set to 0px.

• If the "new selection" in selected and you draw something, you can only make a new selection, deleting the old one.
• If the "add to selection" is selected, then you can add to the existing selection.
• If the "subtract from selection" is selected, then you can subtract from the selection.
• If the "intersect with selection" is selected you can find only intersected part after the second selection.

Try it, you can easily understand. "M" is the shortcut to go to the marquee tool in the tool box. Pressing Shift+M allows you to toggle between the rectangular marquee tool and elliptical marquee tool. To go to single-row/ single-column marquee tool, I don't think there is a shortcut.